On average when full grown Phelsuma quadriocellata quadriocellata will be 4.3-4.7 inches. Males get bigger than females and normally have more brilliant coloration. To keep a pair, I would go no lower than a 20 gallon tank that is manufactured or customized to be taller than wider.
If I figure out how to attach pictures I will send some of my male Ph quad quad and also a pic of some of my terrariums that show a mfg'd and customized tank.

-Adrian