The biggest adult puffer I've ever seen that was raised in an aquarium was 16 inches. A 20 inch puffer was probably caught that size in the wild. I get all of my information from wet web media and they all agree a 90 gallon is good for an adult puffer if there are few tank mates and a 125 is ideal FOR A HEALTHY, LONG, LIFE. The same is said by the team at saltwaterfish.com, since I emailed to ask them before buying my puffer.
Puffers are not tangs, they don't need the swimming room that fish like those do. They are much like lions. The bigger the tank the better of course, but saying a puffer can't live a long happy life in a 125 is wrong. Feel free to back it up with any info other than "they get really big and need a big tank."
That big puffer is awesome, do you have any other pics fo it? I love their faces and attitudes. Mine is about 6-7 inches now and I've never had a fish that is so sociable to people. Always right up on the tank wanting food and never skittish.