They've been documented at almost 3 feet, but not sure how likely they are to hit that in captivity.
My biggest one is around 15 inches and he's thick compared to what you normally see at fish stores, but they stay very thin and snakelike. They seem to be really slow growers, I haven't noticed much growth in any of mine over several months. My biggest one has grown maybe an inch over 8-9 months.
I have a pic of my biggest one but imageshack is being stupid so I guess I'll post it later