I don't like small tanks for animals, really. A ten gallon tank usually costs around $10, another ten for a lid (or have some glass cut), and that'll work pretty well for most terrestrial species.
I suppose some smaller terrestrial Ambystoma might do okay in the six gallon tank - I used to keep them in plastic shoe boxes, which are smaller, but the problem with small tanks is that there's less room for error. If it gets too hot, the animal can't burrow in deeper, and if you miss a misting, the whole thing can go dry before you notice what happened. There's less room for microhabitats in the enclosure too; places that would be more or less humid or hot, etc., that would allow the animal to regulate itself.
Instead of buying an animal for the tank, why not buy the tank for the animal? That way you can look at the species you want, and then pick the right size and type of tank that would be best for it.