I have a tokay gecko in a cage of the exact same size (I think) and those are one of the largest geckos. Very easy to keep but tend to be mean and impersonable. Kinda pushin it for an adult (10-15 inches) but I plan to upgrade. The point is, if it can fit a tokay, you can stick an arboreal gecko in there with maybe the exeption of giant leaftails. Leopard geckos and fattails can do fine in a 10 gallon+ cage so it should be fine (sounds like a 20 high). If you have a specific gecko in mind, just post the name and we can tell you if it will work.