#6 birdshot. The same as you hunt bunnys with. It won't go through a wall and hurt a neighbor or family member in the same manner a slug or Buckshot will.
If you have any doubts about birdshot, go fire a shell or two into a gallon jug filled with water at 7 yards or less. Buck and slugs are TOO powerful to use inside your home. Birdshot is fine. Test it.