You'll want probably at least a 3'x2'x18" enclosure for her as an adult (a bit bigger might be better), and she could reach 6 feet in length, though she should remain relatively slender.

Obviously next time, you should take more care to be sure of what you're buying, but on the plus side, all rainbow boas have that awesome iridescence, so at least you can enjoy having a pretty snake for as long as you've got her. I don't want to get into trouble with the mods over recommending another forum, but in this instance, hopefully they'll forgive me, as it's in the interest of helping someone out: the folks over on the kingsnake.com rainbow boa board are, by and large, friendly, knowledgeable, and helpful. If you have some questions about care that you can't find the answers for here, I'd suggest posting over there; several well-known rainbow breeders hang out there, and if you've got husbandry questions, they can set you straight.