There was a picture posted by one of the regulars on here some time ago of a full grown one set next to a 10 gal tank. It was basically as long as the tank.
I have a group of 5 in my 240 gallon and they are all around 10 inches. The two biggest are 11 or 12 inches. They are monsters and they should not be kept in anything less then a 100 gallon tank IMO and no less then 3 of them.