The general rule of thumb for enclosures is a footprint that adds up to at least the length of the snake. Most BPs, even females, will only get about 5'... a 4'x2' enclosure should be enough floor space for a 6' snake. I currently have T10 and T11s. My largest snake is a BCA that is ~5.5' long and he has plenty of room to roam in his T11. he has 2 large RBI hides, a cat litter pan for a water bowl (large enough to soak in) and a climbing branch. That said, I did opt to get the basking shelf option on all my cages and I will often see them stretch out on the shelf to it's full length. I think if you want to give more room, the shelf would help.

That said, a 4'x30" footprint cage would give you more room for positioning hides differently and decor.