Maybe there's another way to resize the canvas that you're using, but the standard way, the menu option Canvas Size... has an option to resize some or all layers (defaults to none): http://imgur.com/Yk0oaKx Just an FYI.
Oops, not sure how I missed that. Resizing all the layers (maybe lazily until the user tries to draw on them, if that's such a waste when you have a 100 layers or something) should definitely be the default, though. I'd wager that's what people want 99% of the time.
