Minecraft wiki entry for Drowned_farming, associated text: The water in the spawning platform must be at least 24 blocks from the killing platform, because mobs don't spawn nearer than 24 blocks from the player. Make it as large as you want. it can be as small as 2×2 but larger means more drowned. Flowing water is used to sweep mobs into a hole. A small spawning room can sweep the mobs to a hole in the side or corner of the room. A large spawning room, say with at least a 16×16 surface, would have water sweeping the mobs into a hole in the center.