Did you still need help with this? I sort of stumbled over a solution.
11x14 paper is used nowhere but in the US; probably why it's not a standard Apple size. You'll need to be sure you're running the Canon printer driver, as opposed to the Bonjour or a generic driver. Then, open any image in the Preview app, select File -> Print from the menu, and "Show Details" in the Print Options.
From there, where you would normally select a paper size, instead select "Manage Custom Sizes" (I think that was the wording).
Now you can create a new, user-defined paper size (change the title from "Untitled" to "11 x 14"). Afterwards, you'll be able to select this paper size from any app you choose.
Hope it helps? Best wishes