The settings are 31 blocks by 31 blocks, in a square radius and will protect you all the way up into the sky and all the way down to bedrock. This will make your base invincible and this will protect against bears but not against zombies. To protect your base against zombies you will need to upgrade your walls to iron or steel for better defenses. Adding spikes to a kill pit or trench also helps out a lot with defenses too.