Your rabbit will appreciate a cage where it can be able to have enough room and comfort to relax, eat, and do his business inside. The right sized cage also depends on the size of your rabbit. A Dwarf rabbit or similar sized breeds should have a cage measuring at least 36 by 24 inches. Larger breeds will feel more at home in cages measuring at least 36 by 30 inches. 

However when in doubt, bigger is always better. A rabbit will want to be able to fully stretch out or stand on its hind legs at times, so this should be kept in mind.