Might I suggest that "graphic design" simply means "use the image you have to create a design that WILL work."
If you have photo editing software, then you can create a canvas of the right size. Start with a canvas no smaller than the minimum suggested recommendation, and then start designing. The art is using your images on a product in a way that fits the product's image specs. Your images by themselves do not have to be the whole duvet canvas size - the images can be one element of the greater design. That's how I am doing it.
Think book cover design, except switch mental gears to BED COVER design. Look at pictures of duvet covers being sold on home furnishing websites to get some ideas. Many of these are pretty plain and generically lacking in pop.. I do not know that you want to get too wild with the boldness, however ... maybe for a kid's room or a really "out there" person's theme room. You might want the colors to be a bit more subdued and neutral to blend with the overall furnishings of a room. Or NOT.
Find your own sense of balance with how bold you might get.
You have to get design-specific, ... product-specific. Click up a duvet image on your computer screen, ... get a ruler, ... and measure the over run past the duvet borders. Get out your calculator and figure out a good base canvas size to start with. I will not tell you exactly how to do it, because I believe that those of you with a designer's obsession will figure it out. Doing so is good exercise.
