Tenngun pretty well nailed it.
Other than special application and what local game laws will allow, mostly personal preference, maybe recoil tolerance could be a factor, as far as bore size is concerned.
Generally smmothbores perform better with longer barrels.
My 20 gauge smoothbores are legal for any game where I live, depending on how I load it. I might go bigger if I were still into duck hunting. I'd like to get something small, like Britsmoothy's smoothbore. 45, to play with for squirrels, but that would be in addition to my general purpose 20 guage, very versatile.