There is actually a great debate over this subject on PCF. Jools (the owner) and Shane have never seen one over 12 or so inches in the wild. These guys often go to SA on collection trips. But have yet to see a paradallis (sp) or Common Plec as large as the claims are. Nor has anyone been able to provide proof (a pic) that they get larger than this.
I have a monster at about a foot. I've seen a huge probably about 2 foot one at my LFS, it was an albino, really creepy. I'm assuming he was a common one, but I'm no pleco expert.
I saw one last year at a LFS that was between eighteen and twenty inches long. Someone had brought it back, of course. At least the lfs guy believed it was a common, and it did look like one, but I'm certainly no pleco expert. :/
the ASPCA in my area was just brought a common pleco in a 75g tank that is about 18" long. hes a monster, the first thing you see now when you walk in. beautiful pattern though!