I've been looking into size for if (more like when) I get one and the research I concluded on was they'll max at 4-6ft. From what I read they normally average 4-5ft in the wild. Now I have had other people say they'd get to the 9-12 ft range but I just can't see that happening in captivity
Very few get lucky to live long enough to get to 8'-10' mark. Everything caught over 10' is almost always a reference to 100-200 years ago and in my severely limited understanding, these wels must have been 50-100-? years old at that point already. There could be some gene pool erosion or gene freaks too. Humans can reach 8' but very few do and most are between 5' and 6'. Maybe the same applies to the contemporary wels.
But 16' is a legitimate record that most likely will never get repeated and even approached even if only due to overfishing and pollution and habitat destruction of both wels and its food. Same with 15' arapaima, 15' alligator gar, 9' piraiba, 6' RTC, and 9', 700 lbs Mekong Giant Catfish.