Looks like pretty conclusive proof to me that Franco's arms were a tad over 18 inches. But bear in mind that he was only 5'5" or so with those sort of measurements.
If you were 5'10" (or any other average sort of height) you would probably need your arms to be between 19.5 to 20 inches to look comparable to Franco's. Any you would also need to be pretty low in bodyfat as well.
So all things considered, Franco's arms at 18in+ probably isn't too bad.