Yes, that is a good size for them while they are growing. That way they have an easier time finding food. You could probably leave them in there for a couple of months, I'm not sure how fast gray tree froglets grow! I've had my spring peeper froglets in a 10 gallon for 4 weeks so far and they aren't near big enough yet for the next size tank...though they stay MUCH smaller than grays anyways.