Minecraft wiki entry for Multiplayer_details, associated text: A player with no spawn point set (such as by a bed) will be placed randomly at the highest point in a 20x/20z square centered on the x-/z- corner of the spawn point; that is, the 20x/20z spawn square's edges are at SpawnX -10/+9, SpawnZ -10/+9. The 10 block radius can be changed using /gamerule spawnRadius though. Changing the spawn radius also affects the radius of spawn protection. The spawn protection will be removed if the spawnRadius is set to 0.