A baby BP can easily be kept in a 10-gallon. You want your hides to fit your snake snugly. BPs want to be touched on all sides by their hide so they feel secure. I like plastic dollar-store cereal bowls with an opening cut out. Easy to get two of and they'll hold your snake until they're 200-300 grams easily, maybe a bit more if they like wearing their hide as a hat. You only need one water bowl.

Have any pics of your setup? How are you heating, how are you regulating that heat, and how are you measuring your temps?