Their max size is 3", not counting the tail. The places that claim Figure Eights grow to 6" may be mixing up their puffer species. It's quite common for Figure Eights (T. Biocellatus) to get mixed up with T. Nigrovirdis and T. Fluviatilis (GSPs and Ceylons).
Puffers generally grow the fastest when they are young, which is why it is so important to have a properly sized tank before purchasing; even if the puff seems tiny. It's a possibility he could have been stunted, but 6 months is not a long enough amount of time to judge that. These guys don't just catapult from 1" to 3" in a few months.
It is a common myth that fish grow only as big as the tank you give them. It's quite convenient for people to think that fish are magical creatures that may grow 3' in the wild, but if they really want one they can stick it in a 10 gallon and it will stay 5"... The reason fish don't grow to their full potential size in tanks that are too small for them is because they are being stunted and die prematurely. Fish should always grow to the size the grow in the wild, or maybe even bigger because they have a constant food supply, clean water, and no predators. I feel that myth is perpetuated by pet stores to increase their sales...
I can't tell you anything definite about growth rate, but I can tell you my own Figure Eight that I have owned about 1 1/2 years (housed in a 15 gallon or larger, at 1.005-8 SG) was 1.5" when I got her, and now she is about 2.5". It's important to keep in mind that water conditions and water quality also affect the health and therefore the growth of puffs, as well as how complex their tank decor is.
