I presume from your post that you have researched this puffer already and know that it must be kept alone and is very aggresive.
Ours is in a tank 5x2x2 and would not recommend anything less than that size as he is very active and puffers are very messy so it is important to keep the water good.
A great fish to watch, lovely markings and you will no doubt of heard all the funny tales from Sir and Lady Minion about Martha.
It has been said that they need live food but this is not the case, Gomez, along with Martha happily takes prawn, cockle, mussel, whelk etc.
Teeth can easily be kept down with Clams or cockles in the shell and Lady and Sir MInion give Martha the occasional crab, although Gomez has no live food at all.
Good luck if you do decide to get one and if you need any more help just ask.
Gomez is around 13" and hasn`t grown lately so there is conflicting evidence to say what size they reach.
Clare