From my understanding, if your enclosure is too big, you'll just need more hide boxes to make it feel secure. If you stick with the one hide box on the warm side, and one on the cool side in an enclosure that's too large for the animal, it will feel insecure. A 20g tank is plenty large for a growing BP. Once they get to the adult phase of life, then consider building an enclosure based on the length of the full grown animal. The common formula I've seen is that the snake needs to fit across and half way back. i.e. a 6 foot snake would need a 4.5 foot wide enclosure. (Just what I've read, I'm keeping my exisiting normal in a 20g wide tank right now, and the 4 females on order are awaiting the rack I'm getting for them.)

Hope this helps,