I am not sure how big they get. Mine was about 3" long and he was in a 75. He swam all the time, never stopped moving. I found this on the net regarding them.
Spiny Box Puffer
(Chilomycterus schoepfi)
Quick Stats
Minimum Tank Size: 70 gallons
Care Level: Difficult
Tank Conditions: 72-78°F; sg 1.020-1.025; pH 8.1-8.4; dKH 8-12
Temperament: Semi-aggressive
Max. Size In Aquarium: Up to 10"
Reef Compatible: No
Diet: Carnivore
Compatibility: View Chart
Origin: Caribbean
Family: Diodontidae
"The Spiny Box Puffer is also known as the Striped Burrfish. It has a tan to yellow body with dark striping, and short, fixed spines which help protect it from larger, more aggressive fish. Its teeth are actually a fused beak-like structure.
A 70 gallon or larger aquarium is suitable, provided it is a fish-only tank with plenty of room for swimming. It is known to nip at slow-moving fish and long-finned fish, but is not aggressive with others in its own family. It will eat invertebrates found in a reef tank.
The Spiny Box Puffer needs a varied diet of meaty foods including; squid, krill, clams, and hard shelled shrimp to help wear down their ever growing teeth."
I do not agree with the care level, as they are not difficult to keep. They do need lots of swimming room and you need to make sure to feed a variety of foods and watch what other tank mates you put with him.
I didn't find a max size, but I will say I have never seen one over 5".
Hope this helps!