are you talking about striped puffer (A. manilensis)? if you are....they grow to 12" in the wild, but more likely....considerably smaller in captivity (6-7"). you might be able to keep him for a few months, even so you will have to work extra hard to maintain a good water quality. they do grow fast and messy eaters. if you have to returm him in a few months, why not do so now? in this way, you won't get attached to your puffer too much. or do you plan on upgrading to a bigger tank in the near future?