A nice Colombian-locality boa (Boa constrictor imperator) male is typically going to stay in the 5-8' range, with 6' being common. When you do see a bigger male, it's usually an older snake (5+ years). You're not suddenly going to have a giant snake on your hands overnight, especially if you feed on an appropriate schedule (i.e. 1x every 7-10 days). There are so many fantastic breeders working with Colombian boas that you should be able to find something that you like, from someone respectable, in a reasonable price range.

Also, Eco Publishing (http://www.reptileshirts.com) just released a lovely book called Boas in Captivity, written by Vin Russo, who is an esteemed boa keeper/breeder/enthusiast, as well as an upstanding member of the herp community. We just got several copies for our retail store last week, and it's a thoroughly enjoyable read. For under $10, it's a herp-library-find that no boa keeper should be without!



K~