Green Iguana question?

I just purchased a Green Iguana. She is tiny right now and I have her in a 20 gallon. I realize it is too small but I wanted to do some research before purchasing something larger next week. The people at the pet store are clueless so I was wondering what to house it in. I know they like to climb and I have seen those giant terrariums that are tall rather than long and have trees and stairs, I just wasn't sure if that was right or if I should just use a 55 gallon for a while. I just don't want to spend $200 on a tank if in 6 months I will need to purchase something larger. How fast do they grow and would it be worth buying a 55 gallon now and a custom or larger one later or should I just go for the giant one right away?