diff --git a/apps/client/src/app.tsx b/apps/client/src/app.tsx
index da5687c..63f5e75 100644
--- a/apps/client/src/app.tsx
+++ b/apps/client/src/app.tsx
@@ -45,8 +45,8 @@
       <LobbyScene
         room={room}
         myId={myId}
-        onStart={(targetPlayerCount) =>
-          socketManager.lobby.startGame(targetPlayerCount)
+        onStart={(payload) =>
+          socketManager.lobby.startGame(payload)
         }
         onBackToTitle={() => returnToTitle({ leaveRoom: true })}
       />
diff --git a/apps/client/src/network/handlers/LobbyHandler.ts b/apps/client/src/network/handlers/LobbyHandler.ts
index 1b1d70e..00c037b 100644
--- a/apps/client/src/network/handlers/LobbyHandler.ts
+++ b/apps/client/src/network/handlers/LobbyHandler.ts
@@ -5,7 +5,7 @@
  */
 import type { Socket } from "socket.io-client";
 import { contracts as protocol } from "@repo/shared";
-import type { ServerToClientPayloadOf } from "@repo/shared";
+import type { ServerToClientPayloadOf, StartGameRequestPayload } from "@repo/shared";
 import { createClientSocketEventBridge } from "./socketEventBridge";
 
 /** ロビー画面で利用する通信操作の契約 */
@@ -25,7 +25,7 @@
       room: ServerToClientPayloadOf<typeof protocol.SocketEvents.ROOM_UPDATE>,
     ) => void,
   ) => void;
-  startGame: (targetPlayerCount?: number) => void;
+  startGame: (payload?: StartGameRequestPayload) => void;
 };
 
 /** ロビー画面向けのソケットハンドラを生成する */
@@ -43,10 +43,8 @@
     offRoomUpdate: (callback) => {
       offEvent(protocol.SocketEvents.ROOM_UPDATE, callback);
     },
-    startGame: (targetPlayerCount) => {
-      emitEvent(protocol.SocketEvents.START_GAME, {
-        targetPlayerCount,
-      });
+    startGame: (payload) => {
+      emitEvent(protocol.SocketEvents.START_GAME, payload ?? {});
     },
   };
 };
diff --git a/apps/client/src/scenes/lobby/LobbyScene.tsx b/apps/client/src/scenes/lobby/LobbyScene.tsx
index af8cc78..ae26629 100644
--- a/apps/client/src/scenes/lobby/LobbyScene.tsx
+++ b/apps/client/src/scenes/lobby/LobbyScene.tsx
@@ -1,12 +1,14 @@
 import { useEffect, useMemo, useState } from "react";
 import { domain } from "@repo/shared";
+import type { FieldSizePreset, StartGameRequestPayload } from "@repo/shared";
+import { config } from "@client/config";
 import { OVERLAY_BUTTON_STYLE } from "@client/scenes/shared/styles/overlayStyles";
 import { LobbyRuleModal } from "./components/LobbyRuleModal";
 
 type Props = {
   room: domain.room.Room | null;
   myId: string | null;
-  onStart: (targetPlayerCount: number) => void;
+  onStart: (payload: StartGameRequestPayload) => void;
   onBackToTitle: () => void;
 };
 
@@ -46,6 +48,13 @@
   const [selectedStartPlayerCount, setSelectedStartPlayerCount] = useState(
     minimumStartPlayerCount,
   );
+  const fieldPresetOptions = useMemo(() => {
+    return Object.keys(
+      config.GAME_CONFIG.FIELD_PRESETS,
+    ) as FieldSizePreset[];
+  }, []);
+  const [selectedFieldSizePreset, setSelectedFieldSizePreset] =
+    useState<FieldSizePreset>(config.GAME_CONFIG.DEFAULT_FIELD_PRESET);
   const [isRuleModalOpen, setIsRuleModalOpen] = useState(false);
 
   useEffect(() => {
@@ -63,7 +72,24 @@
   }, [minimumStartPlayerCount, maxStartPlayerCount]);
 
   const handleStart = () => {
-    onStart(selectedStartPlayerCount);
+    onStart({
+      targetPlayerCount: selectedStartPlayerCount,
+      fieldSizePreset: selectedFieldSizePreset,
+    });
+  };
+
+  const toFieldPresetLabel = (preset: FieldSizePreset): string => {
+    const range = config.GAME_CONFIG.FIELD_PRESETS[preset].recommendedPlayers;
+    const baseLabel =
+      preset === "SMALL"
+        ? "小"
+        : preset === "MEDIUM"
+          ? "中"
+          : preset === "LARGE"
+            ? "大"
+            : "極大";
+
+    return `${baseLabel} (${range.min}-${range.max}人目安)`;
   };
 
   return (
@@ -226,6 +252,40 @@
                     ))}
                   </select>
 
+                  <label
+                    htmlFor="field-size-preset"
+                    style={{
+                      fontSize: "0.95rem",
+                      fontWeight: 700,
+                      textShadow: "1px 1px 2px rgba(0,0,0,0.7)",
+                    }}
+                  >
+                    フィールドサイズ
+                  </label>
+                  <select
+                    id="field-size-preset"
+                    value={selectedFieldSizePreset}
+                    onChange={(event) => {
+                      setSelectedFieldSizePreset(event.target.value as FieldSizePreset);
+                    }}
+                    style={{
+                      width: "100%",
+                      padding: "10px 12px",
+                      borderRadius: "8px",
+                      border: "1px solid rgba(255,255,255,0.4)",
+                      background: "rgba(0,0,0,0.55)",
+                      color: "white",
+                      fontSize: "1rem",
+                      fontWeight: 700,
+                    }}
+                  >
+                    {fieldPresetOptions.map((preset) => (
+                      <option key={preset} value={preset}>
+                        {toFieldPresetLabel(preset)}
+                      </option>
+                    ))}
+                  </select>
+
                   <button
                     onClick={handleStart}
                     style={{
diff --git a/apps/server/src/application/coordinators/startGameCoordinator.ts b/apps/server/src/application/coordinators/startGameCoordinator.ts
index 1d09b4a..c1fd7b4 100644
--- a/apps/server/src/application/coordinators/startGameCoordinator.ts
+++ b/apps/server/src/application/coordinators/startGameCoordinator.ts
@@ -3,6 +3,7 @@
  * START_GAMEイベントの調停を行い，ルーム状態更新とゲーム開始処理を橋渡しする
  */
 import { type StartGameOutputPort } from "@server/domains/game/application/ports/gameUseCasePorts";
+import type { FieldSizePreset } from "@repo/shared";
 import type { StartGameCoordinatorDeps } from "./coordinatorDeps";
 import { startGameUseCase } from "@server/domains/game/application/useCases/startGameUseCase";
 import {
@@ -19,6 +20,7 @@
 type StartGameCoordinatorParams = {
   ownerId: string;
   requestedPlayerCount?: number;
+  requestedFieldSizePreset?: FieldSizePreset;
 } & StartGameCoordinatorDeps & {
     output: StartGameOutputPort;
   };
@@ -27,6 +29,7 @@
 export const startGameCoordinator = ({
   ownerId,
   requestedPlayerCount,
+  requestedFieldSizePreset,
   roomManager,
   runtimeRegistry,
   output,
@@ -73,6 +76,7 @@
     roomId: updatedRoom.roomId,
     socketId: ownerId,
     totalPlayers: updatedRoom.players.length,
+    fieldSizePreset: requestedFieldSizePreset,
   });
 
   const humanPlayerIds = updatedRoom.players.map((player) => player.id);
diff --git a/apps/server/src/network/handlers/game/gameEventOrchestrators.ts b/apps/server/src/network/handlers/game/gameEventOrchestrators.ts
index faad83c..88c3ba5 100644
--- a/apps/server/src/network/handlers/game/gameEventOrchestrators.ts
+++ b/apps/server/src/network/handlers/game/gameEventOrchestrators.ts
@@ -4,7 +4,7 @@
  * 受信ハンドラからユースケース実行責務を分離する
  * ランタイム未解決時はNetworkスコープでignored_missing_roomを記録する
  */
-import { contracts as protocol, domain, type BombHitReportPayload, type PingPayload, type PlaceBombPayload } from "@repo/shared";
+import { contracts as protocol, domain, type BombHitReportPayload, type PingPayload, type PlaceBombPayload, type StartGameRequestPayload } from "@repo/shared";
 import { readyForGameCoordinator } from "@server/application/coordinators/readyForGameCoordinator";
 import { startGameCoordinator } from "@server/application/coordinators/startGameCoordinator";
 import { movePlayerUseCase } from "@server/domains/game/application/useCases/movePlayerUseCase";
@@ -20,9 +20,7 @@
 } from "@server/network/types/connectionPorts";
 
 /** START_GAMEイベントの入力ペイロード型 */
-export type StartGamePayload = {
-  targetPlayerCount?: number;
-};
+export type StartGamePayload = StartGameRequestPayload;
 
 /** PINGイベントの入力ペイロード型 */
 export type PingEventPayload = Parameters<typeof handlePingEvent>[1];
@@ -63,6 +61,7 @@
   startGameCoordinator({
     ownerId: deps.socketId,
     requestedPlayerCount: payload.targetPlayerCount,
+    requestedFieldSizePreset: payload.fieldSizePreset,
     roomManager: deps.roomManager,
     runtimeRegistry: deps.runtimeRegistry,
     output: deps.output,
diff --git a/apps/server/src/network/validation/socketPayloadValidators.ts b/apps/server/src/network/validation/socketPayloadValidators.ts
index b999b69..e5d3538 100644
--- a/apps/server/src/network/validation/socketPayloadValidators.ts
+++ b/apps/server/src/network/validation/socketPayloadValidators.ts
@@ -57,21 +57,47 @@
 /** START_GAMEイベントのペイロードが開始要求情報であるか判定する */
 export const isStartGamePayload = (
   value: unknown,
-): value is { targetPlayerCount?: number } => {
+): value is { targetPlayerCount?: number; fieldSizePreset?: string } => {
   if (typeof value !== "object" || value === null) {
     return false;
   }
 
   const candidate = value as Record<string, unknown>;
   const targetPlayerCount = candidate.targetPlayerCount;
+  const fieldSizePreset = candidate.fieldSizePreset;
+
   if (targetPlayerCount === undefined) {
+    if (fieldSizePreset === undefined) {
+      return true;
+    }
+
+    return (
+      fieldSizePreset === "SMALL"
+      || fieldSizePreset === "MEDIUM"
+      || fieldSizePreset === "LARGE"
+      || fieldSizePreset === "XLARGE"
+    );
+  }
+
+  const isValidTargetPlayerCount = (
+    typeof targetPlayerCount === "number" &&
+    Number.isInteger(targetPlayerCount) &&
+    targetPlayerCount > 0
+  );
+
+  if (!isValidTargetPlayerCount) {
+    return false;
+  }
+
+  if (fieldSizePreset === undefined) {
     return true;
   }
 
   return (
-    typeof targetPlayerCount === "number" &&
-    Number.isInteger(targetPlayerCount) &&
-    targetPlayerCount > 0
+    fieldSizePreset === "SMALL"
+    || fieldSizePreset === "MEDIUM"
+    || fieldSizePreset === "LARGE"
+    || fieldSizePreset === "XLARGE"
   );
 };
 
diff --git a/packages/shared/src/config/gameConfig.ts b/packages/shared/src/config/gameConfig.ts
index ee75165..e178d72 100644
--- a/packages/shared/src/config/gameConfig.ts
+++ b/packages/shared/src/config/gameConfig.ts
@@ -11,6 +11,40 @@
   HURRICANE_ROTATION_QUANTIZE_SCALE: 4,
 } as const;
 
+/** AOI同期で利用する1セルのグリッド幅 */
+const AOI_CELL_SIZE = 3 as const;
+
+/** フィールドサイズ種別ごとのAOIセル数と推奨人数レンジ */
+const FIELD_PRESETS = {
+  SMALL: {
+    aoiCols: 8,
+    aoiRows: 8,
+    recommendedPlayers: { min: 4, max: 20 },
+  },
+  MEDIUM: {
+    aoiCols: 12,
+    aoiRows: 12,
+    recommendedPlayers: { min: 20, max: 40 },
+  },
+  LARGE: {
+    aoiCols: 15,
+    aoiRows: 15,
+    recommendedPlayers: { min: 40, max: 70 },
+  },
+  XLARGE: {
+    aoiCols: 18,
+    aoiRows: 18,
+    recommendedPlayers: { min: 70, max: 100 },
+  },
+} as const;
+
+/** 既定で利用するフィールドサイズ種別 */
+const DEFAULT_FIELD_PRESET = "MEDIUM" as const;
+
+const defaultFieldPreset = FIELD_PRESETS[DEFAULT_FIELD_PRESET];
+const defaultGridCols = defaultFieldPreset.aoiCols * AOI_CELL_SIZE;
+const defaultGridRows = defaultFieldPreset.aoiRows * AOI_CELL_SIZE;
+
 /** ゲーム全体で利用する共有設定値 */
 export const GAME_CONFIG = {
   // ゲーム進行設定（クライアント/サーバー契約）
@@ -24,9 +58,14 @@
   HURRICANE_POSITION_QUANTIZE_SCALE: NETWORK_SYNC_CONFIG.HURRICANE_POSITION_QUANTIZE_SCALE, // 後方互換のため維持
   HURRICANE_ROTATION_QUANTIZE_SCALE: NETWORK_SYNC_CONFIG.HURRICANE_ROTATION_QUANTIZE_SCALE, // 後方互換のため維持
 
+  // AOI設定（クライアント/サーバー契約）
+  AOI_CELL_SIZE,
+  FIELD_PRESETS,
+  DEFAULT_FIELD_PRESET,
+
   // グリッド（マス）設定（クライアント/サーバー契約）
-  GRID_COLS: 40, // 横のマス数（グリッド単位）
-  GRID_ROWS: 40, // 縦のマス数（グリッド単位）
+  GRID_COLS: defaultGridCols, // 横のマス数（グリッド単位）
+  GRID_ROWS: defaultGridRows, // 縦のマス数（グリッド単位）
 
   // プレイヤー挙動設定（内部座標はグリッド単位、契約値）
   PLAYER_RADIUS: 0.5, // プレイヤー半径（グリッド単位、目安: 0.05〜0.2）
diff --git a/packages/shared/src/protocol/eventPayloads.ts b/packages/shared/src/protocol/eventPayloads.ts
index 8339b07..dca324e 100644
--- a/packages/shared/src/protocol/eventPayloads.ts
+++ b/packages/shared/src/protocol/eventPayloads.ts
@@ -30,6 +30,7 @@
   NewPlayerPayload,
   RemovePlayerPayload,
   GameStartPayload,
+  FieldSizePreset,
   StartGameRequestPayload,
   MovePayload,
   PlaceBombPayload,
diff --git a/packages/shared/src/protocol/payloads/gamePayloads.ts b/packages/shared/src/protocol/payloads/gamePayloads.ts
index f44645e..68139ba 100644
--- a/packages/shared/src/protocol/payloads/gamePayloads.ts
+++ b/packages/shared/src/protocol/payloads/gamePayloads.ts
@@ -104,8 +104,12 @@
 };
 
 /** start-game イベントで受信するゲーム開始要求 */
+export type FieldSizePreset = "SMALL" | "MEDIUM" | "LARGE" | "XLARGE";
+
+/** start-game イベントで受信するゲーム開始要求 */
 export type StartGameRequestPayload = {
   targetPlayerCount?: number;
+  fieldSizePreset?: FieldSizePreset;
 };
 
 /** move イベントで送受信する移動入力情報 */