WOW... nice to see the myths are still bouncing around regarding RTC's... 6' is ALONG Way off.. 4' is not the max however... it is extremely rare for someone to have one that big in a home aquarium... you prob know the most owners of these guys at that size as members on this site. Typically RTC's will have a huge rush to grow to 2' as was mentioned, after that their growth actually does not slow down as most people have been led to believe, it simply changes direction, at 18-24" you will most likely have a leaner sleeker RTC... once they get to that size you will notice them bulking up... their heads will get much wider... their football shape will come in very nicely and you will see that their overall thickness will go up dramatically. Short version is, their weight will continue to climb just as fast as the length did during the first year.
I will leave this conversation as is before I start a novel on these guys