The Designer Help has a topic: Resizing Document and Canvas which has some problems:
First, the before/after images are missing.
Next, the instructions don't make any sense to me:
Designer doesn't have a Document menu.
Photo has a Document menu, and at first I wondered if part of the Photo Help had gotten into the Designer Help, but the operations described don't work in Photo, either.
Photo's Document menu has Resize Document and Resize Canvas, not simply Resize.
Neither of Photo's Resize functions under Document has a Context Toolbar.