Unless it is outside their normal size parameters (i.e. an unusual individual) a regular male Bci (or, common redtail) should stay well under 10'... Typicallly 5-6'. Also, the central americans, mexicans, and some of the island boas stay well under that as well (typically smaller than the length I mentioned for regular Bci males above).

Female boas (common boas) get a LOT heavier and can be longer than males, so I would avoid a female. Also avoid Bcc (true redtails) as they get larger and heavier than common boas.

Boas are great and you should enjoy any of the localities / morphs of a Bci, I would advise getting the snake when it is smaller so that you can "grow with it" and get used to handling it. I would also suggest handling it in person (if possible) or specifically requesting a calm animal from the breeder. I would NOT buy from a reseller... and it is common for resellers to sell boas. My reason behind this is that the actual breeder can give you more information about date of birth, backgrounds on the parents (you can ask for pics and adult lengths on them to see what your boa's potential will be) and many times the breeder will be more willing to work with you in picking the right animal than a reseller would be.

I love my boas and just got my first last year... Now I'm on number 3, so be warned, they are addictive.