My guess....... 24-30" is about it in a "normal" large tank/small pond. They usually slow down at around 18-24" and slowly grow from there. Sure they will grow forever, but the mythical 4' tank raised RTC has yet to be seen.
There is one over 5.25 feet that has been posted that has lived in an Aquarium in Amsterdam since 1972 where it was introduced to the tank at under a foot. I'd say that counts as tank raised.
Ironically, you posted in the thread too. You have have forgotten.
While your average is likely correct I'd say it's mostly because the fish are never given the chance to mature in a tank and either die prematurely or are killed from some kind of problem with their home. (IE; pump failure, lack of water changes, etc)