For us basically it's:

hatchlings up until they reach about 200 grams - 6 qt

200 grams until they hit about 600-700 grams or so - 15 qt

larger than 700 grams - 32 qt for males (all their life)
- 32 qt for females until they are 2,000 grams, after that they go into 41 qts (we do keep our 3 biggest female BP's in bigger than 41 qt tubs)

This is dependent though on the individual snake. We've had a few that when they reached a size to graduate from the 15 into the 32 they absolutely freaked and refused to eat. We put them back into the 15 and left them for another few months and then they did accept the move better. I think for the really shy ones any move up in tub size can be stressful so sometimes you just have to work with what works for that particular snake.

It also depends on whether or not you have hides and how many hides. Most of our bigger BP's don't use hides as the enclosed rack basically acts as their hide. Some do need hides though so they require a tub size big enough to allow for that so they can feel less stressed.