I think 24" is a understatement IMHO. although your correct that 24" is accepted as their normal aquarium size, on average I see them closer to 27-28" and some times 30+" My huge one belonged to my brother who had him a long time and his home was a 850 litre tank ;D
"big ball of light, spits out dinosaurs" - Mac, Primeval New World
Well, it depends on the tank and keeper. But generally I think we can agree that both armatus and Favus max out at about(!) 30"
I'm in a similar age to you and also obsessed with spiny eels.. We have lots in common