I don't think you have to worry about this fish reaching six feet lol. Rarely do you hear about a RTC exceeding much over 3' in your average (or even slightly above average ) private hobbyists aquarium. In the wild they may approach 4.5' -5', but not likely in the home aquarium. As well the marble, I'm assuming you mean Leiarius longibarbis(?!), MAXES out AT 2.5'-3'. Soooo..... I think if you split the difference, you SHOULD end up with an approximately 3' fish given the appropriate size aquarium and care. However, Like necrocanis and Wolf 3101 said, it's probably gonna grow FAST, and it's full adult size IS all guess work right now. I'm sorry you had a bad experience with the seller, I know them to be good and reputable as well, but hey, s*** happens and that's why these forums exist, to get a second opinion! That being said, I think you should raise said catfish to it's full potential and let us all know! lol.
Best of luck!!!