This is and old debate in the fancy and seems to have as many answers as it does people responding to it....however as a past and current RTC owner I can tell you that in almost every case the responces are usually grossly inflated.
Yes RTC's do in fact swim around from time to time but for the most part they are content to pick a spot on the bottom of the tank and hang out until mealtime.
Yes they do in fact get fairly large but unless your 'powerfeeding' them and doing twice weekly water changes you'll rarely ever see one larger than 3 feet long in a home aquarium.
Yes they do grow fast...so much so as babies that you can almost watch it happening but this fantastic growth rate slows down a LOT once they reach about a foot long or so.
There is a vast differance between 'stunting' a fishes growth and artificially stimulating the growth rate. I know of RTC's who have lived long and very happy lives in anything from a 180 to a 300 gallon tank. Personally I prefur to see them in at least an 8 foot long tank.