As for enclosure sizes, I am a big fan of large naturalistic enlcosures for all my caged pets.

However, with shy animals like BPs you have to be extremely careful to make sure the setup gives them the security they need. Lots of hides, visual barriers, etc.
I see that my snake utilizes all the space at night when I catch him all stretched out in it and it makes me happy I can give him the room he needs to do that. With a large enclosure I can have warm and cool water dishes, both large enough to bathe in comfortably if desired. The large enclosure also will provide plenty of room to get safely away from a regurge if *god forbid* that ever happens.

However, I keep an appropriately sized rubbermade tub sterilized and ready to house the snake should he ever have some kind of crisis that warrants a small, controlled environment.

Aim for your best, but prepare for the worst.