I think trial and error might work best. You want more than one hide in there anyway (at least one on each side), so if you start with some cheap hides of different sizes (get a few different sized butter tubs, etc) and that way once you get your BP home you can decide right then which ones to use. My BP likes the coconut hides you get in petstores. They come pretty small, so assuming you're starting with a young one those might be a good start. It's funny, if I go to lift it off him he gets stuff in it.