The local vet I got into contact with to make sure there was some experience locally if I ever have problems explained a little bit on enclosure size to me. She said that you ideally want to make sure that the length and width combined are not too much shorter than the length of your BP fully stretched out, especially if they are younger and growing fast. She said that if they can't regularly stretch out as they are growing and developing that it could lead to their rib cages not developing properly. I can see this making it difficult with BPs since they stress sometime if the enclosure is too large. Based on what I have gathered on this site and a couple others, I would say going bigger is better. Someone else mentioned larger tubs with extra clutter. It's not very difficult to make a 28qt tub seem much much smaller to your BP.
As the OP mentioned it's probably not wise to keep a 400g BP in a 6qt tub. I have my Pastel that was only 123g last week before feeding in a 34qt Hefty Storage bin, the same as my 280g normal. With a small dog dish for water, 2 hides and a big rock there is very little excess space.
This is what I have both of mine in
http://www.hefty.com/products/home-s...efty-34qt.aspx
I just recently purchased a 41qt Sterilite for a possible 3.5ft female rescue I want to adopt.