Can I ask why he would need an arboreal cage? You can keep boas in an arboreal cage---they do enjoy climbing from time to time---but they need plenty of floor space, as well. Unless you're building one yourself that can provide both length and height, your best bet is sticking with a longer cage rather than a taller one.

A bigger cage is never a bad idea with adult boas. I would say a 4 footer would work just fine if she's already 5-6 years old, but if you want to stay on the safe side, a 6' will do you well indeed.

I agree with giving Rich a call... a stellar guy. You can tell him what you want, what you're thinking about in the future, and he can tell you what he can do.