One Size Fits All Cage?

[Coluber42]

You'll get differing opinions on what an adult BP needs, but I see no reason to put the animal in a 36" cage if you can afford or have room for a 48" one. If it is appropriately filled with hides, plants, tubes, branches, platforms, whatever, the snake will be able to feel secure when it wants to hide, and really will use what space you give it; just maybe not at the times of day when you're watching. I also prefer a taller cage; they'll climb on branches and upper platforms if they're provided, and it's good exercise and enrichment.
For a juvenile, a larger cage can still be fine as long as it's full of lots of hides, fake or real plants, branches, whatever other stuff. The snake should be able to get from one side to the other basically without being seen if desired.

If you get a youngster who has only lived in tubs, one way to ease the transition could be to get a tub and put it inside the larger cage, with a hole in the lid. Then the snake can take its time adjusting to the new environment and still retreat to the tub if it wants.