I have to dig up some pics of my 3' RTC that died due to a weekend power outage at the school i teach at. He was close to 8 years old. I have grown a few to the 30-36" mark but never any bigger. Ironically my largest one was grown in my smallest tank, a 150 wide which has the same footprint as a 180 just 4" shallower. I currently have 3 RTC's that are but juvi's in the growing stages, with the biggest one not even a foot yet. I kept a beautiful 30+ incher in my 2500 gallon indoor pond for about 3 years and he stopped growing at 32". He was healthy as a horse, and as wide as a school bus, but just stopped growing. I'm always a little skeptical of the supposed 4-5' size some say, In a 300 gallon tank or a thousand gallon pond i dont see this being the norm.
Just my experience.
bob