I wouldn't put a green tree in there. Or a newt. I would choose either itty bitty frogs such as reed frogs, or an invert. Giant millipedes are nice, and don't need much space considering their size.
There seems to be a lot of regional variation. My local females are typically over 2", some over 2.5", while populations I've seen in the Outer Banks are tiny, seldom over 1.5". I've got a trio of greens in a 20, and they use the whole tank.
Honestly, I would prefer to plant it rather than use it as an animal habitat. A few little ferns and Cryptanthus would be nice. You could add some locally-collected millipedes and isopods for movement.