This is a long shot, since I don't believe it would have any bearing on the actual printed image size. Make sure you have "Paper Size Check" turned on in the Printer Setup menu. I'm now running a P7000, so I'm not sure that's the correct menu in which to find that setting on the 11880, but it's close. Can't hurt to check it at least, and turn it on if it is presently off. For one thing, you will waste about a foot of roll paper/canvas if that setting is turned off.
Gary