Without actually seeing the snake in person, it's really hard to say how big your hides should be....but here's a neat illustration of just how tight they like 'em. Keep in mind, the bowls in this picture are small ones (about 5" across at the base) and most of that space underneath is taken up by the actual water bowl itself.
http://www.ball-pythons.net/forums/s...ad.php?t=35212