That's a great idea, and something that I've vaguely considered. For this particular design, the user saved it at 9.8 x 5.3 inches, so that's the default size

That's a great idea, and something that I've vaguely considered. For this particular design, the user saved it at 9.8 x 5.3 inches, so that's the default size in the design center. I like the idea of immediate size options, that allow you to jump between sizes. Of course, every object (piece of art, text, whatever) is a custom size, so it would have to work based on percentages somehow, and not a set number of inches. We're working on it.