I would probably start out with a smaller enclosure. Easier to regulate temps and humidity.
I'm planning on building or buying some larger cages for my boas also. I never really thought about going over 6 feet long and about 30 inches high.
I really want to have some "show" enclosures. Really like the more natural looking set ups with climbing branches and maybe a few fake plants, etc...
Addition: My biggest female is about 7 foot and is an '07. She'll grow some, but I'd be surprized if she ever got over 8 foot. She's a BCI/Columbian.