I am sure you can maintain a pair of frogs in a ten gallon tank but what a boring life they must have.

Tree frogs in the wild don't have a short leash and the more you can provide in confinement the more entertainment value you will receive from the frogs in which you keep. Height is going to be of most importance here since their living environment in the wild is stories above the ground floor.
Myself, 20 gallons minimum for two tree frogs with the home being higher than it's width is always the plan.