Something related… my lab prints the short side of the image along a fixed width, for example 13 or 20 cm. If I want my image to be X wide or Y tall I also have to add borders elsewhere.
I think it would make sense to specify: image size (locked to aspect ratio), physical size (also locked), and canvas size (by default equal to physical size, but values can be increased).
I think no fill color is needed (but it could be useful for people who want to leave the border on).