Once you pick your species, choose gender carefully. Female pacmans are the giants, and males get only 5 inches or so. The opposite is true with pyxies.
Pacmans are, by far ( at least in my experience ) more interesting than Pixies. Their gluttony is proverbial. Their colours and patterns are wonderfull. Their calm, Jaba the Hut demeanour, exquisite..and they are ( imo ) much more fun to feed....