Just wanted to chime in and say I am happy with all my RBI products, and that I think it is pretty common to keep an adult boa in a 4x2x1 cage. If she turns out to be a monster as far as size, you might end up needing a larger one... but there are a couple different factors that come into play as far as what her length will be when fully grown.

Here's a page that discusses a little about adult sizes:
http://www.boa-constrictors.com/com/sizeofboas.html