Ok, so lets see, the link that inchworm provided does not show measurements, so proves nothing.
The first link that you provided Matchstickman, I could build a site that states it up to 8 feet would you blindly quote that?
The second link says Hypostomus ssp, where it clearly shows photos of Pterygoplichthys gibbiceps, that sites reliability goes straight out of the window for me.
I prefer to work from sites such as www.planetcatfish.com or even www.fishbase.org neither of which quote 24 inches as a maximum size for commons. I should clarify at this point that I consider a common as Pterygoplichthys pardalis or Pterygoplichthys disjunctivus. Hypostomus spp are less frequently seen in the trade these days, and of course generally have a smaller maximum size than Pterygoplichthys spp anyway.
Whilst not disputing that these fish are far too large for most common aquaria (I would class them with RTC and Pangasius and say they shouldn't be sold in the trade) to say 'they reach 24 inches' is wrong imo. I hope the above shows you that you do not in fact 'win'.