There was a previous discussion regarding RTC size. It seems to me that the reason that potentially 5' fish max out at 3'-3.5' is not because their sizes are exaggerated for dramatic effect, but because there is often improper care at some point in the fish's life. Especially if they're wild caught, they're put in bags where waste builds up, then sent to LFSs which sometimes don't take very good care of their fish, then go into homes with tanks too small for them. Not many people bring an RTC or ARTC home and put it directly into a 240, like you did. Smaller sizes are most likely a result of stunting.
I think with proper care an ARTC should at least reach 4'. The rest is up to genetics.