I agree with JaredAren that there is a lot of opinion in what an adequate cage size is.
FWIW, here's my opinion. Carl Kauffeld, in Snakes: the Keeper and the Kept, says that a 3x2 foot cage is acceptable for a six foot long boa. I've owned several six foot long boa constrictors, and they seemed quite happy in a 3x2 foot cage. Especially if they had a hiding box. By the time a boa constrictor reaches seven feet, it's time to start thinking about a bigger cage.
I tend to prefer cages that are 14-20 inches high to cages that are 12 inches high. But that is because most of my cages are front opening, and I have to stick my head and shoulders in to reach the back of a 2 ft deep cage. For a boa constrictor, cage height is determined more by owner preference than snake preference.
Paul Hollander