We have several members who are keeping M. armatus well above 24". I'm not sure where you could be getting your information, but I would stop relying on that source if I were you. As proof, take a look at akskirmish's old armatus:
http://www.monsterfishkeepers.com/forums/showthread.php?359404-My-last-Eel-Pic
As for diferentiating between the two species, a much simpler and more practical method would be to just look at its pattern. A completely solid color on the underside means you have a Mastacembelus armatus; the presence of patterning, which can range from the full "tire track" pattern to just a few spots, on the underside means you have a Mastacembelus favus.