i'll have to agree. i have my green spotted puffer in a 20gal. but hes only an inch long, within the next couple weeks he's going in a 30gal. i'm just waiting on a heater, the tanks already cycled. but they do much better in a larger tank.
if you leave a puffer in a tank thats too small (or any fish) for too long, it will stunt their growth.
and a +1 to jimbo, you should never put a fish you keep in an aquarium back into the wild.
you can also check out The Puffer Forum
you'll find tons of experienced puffer keepers there!