I had a pet Green Tree Frog a long, long, long, long, time ago. He seemed pretty comfortable in a 20 gallon tank.

Just looking at some really, really old photographs right now. It was a highly aquatic environment, with the tank filled almost a third of the way with water. There were two pretty hefty climbing branches, and a good sized dieffenbachia plant in a pot in a ceramic holder. I knew nothing about keeping frogs at the time, and he shared the tank with a couple of newts, and also a couple of bullfrog tadpoles/baby bullfrogs for awhile. He still managed to survive for several years though, and was very tame. He used to sit quietly on my finger, and would sometimes start croaking when he was petted.

I think they're good frogs, and overall, they're pretty forgiving. I wouldn't keep one in too small of a setup, though.