I would recommend about 10 gallons per tree frog when you move them to a larger tank. I have my 4 in a 35 gal hex tank, if it wasn't for the nice vertical space of that tank, it would probably be too small, but it works well.

Here is the spring peeper grow out tank -


I would add in some crisscross branches for the gray tree frogs. They like the bromeliad because it holds water, but not enough to drown them.