Depends on which species. The largest is the Sungazer (Cordylus giganteus) at about two feet, which is waaaaay to big for a twenty. I had an adult pair of much smaller Cordylus tropidosternum in a twenty.
Hrm, any links to pictures of the different types? This is a juvenille and I doubt the keeper giving it away knows much outside of that it's an armadillo lizard.