Like the above poster said, I'd start with a northern locality. I've heard Nicaraguan and Hogg Islands are pretty popular, but I think Tarahumara Mountain boas are the smallest. Gorgeous animals but kind of difficult to find.

As for the size difference (assuming we're talking about the more Southern localities), I've heard males grow to about 8 ft and females to 9. I think the record is somewhere around 12 ft, but I think she was powerfed too.

Seeing as I haven't had a boa yet myself, I wouldn't quote me on any of that, but I've been researching for a few months now.