I agree your BP will likely not utilize a large water bowl. but the bigger the better when you are housing a BP in a tank. keeping the humidity up in a glass tank with aspen can be tricky so I would get the biggest water bowl I could. Aim for more surface area over depth. Also a bigger water bowl will take up more room and perhaps add some security, you want lots of clutter so your BP doesnt feel like it's in a big open space.