Hey welcome to the forum!
For US green tree frogs many recommend 10-gallons, but this is just too small. I wouldn't go below a 20-gallon high. Despite their small size, at night they come quite active and like to explore the tank. An exo terra terrarium that measures 18 x 18 x 24" is well suited for a small group. Very easy species to care for and enjoyable at night to watch with blue LEDs night lights.