My crested lived in a 10 gallon tank that is horizontal. She seemed to be doing fine with that, but everything I've heard/read said they do better with more vertical space, so we bought a wire mesh extension for the top of the cage which makes it probably 17 or 18 gallons and almost double the height it used to be, and she seems to love it - most nights when I come home from work, I find her hanging out (literally) in the top part of it. She uses all the vertical space, from the floor to the very top.
This is the one I have, though I got mine at a local pet store and paid $30, which is significantly less than the place I'm linking to, so I'd shop around for prices if you want to get one, but at least it shows you what I'm using:
http://www.petmountain.com/show_product/509549/
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0.1 Albino Leopard Gecko - Tigger
0.1 Crested Gecko - Pooh-Bear