There's a bit more flexibility when it comes to quilts and comforters because they don’t have to fit as tightly as fitted sheets. You can usually go between one or two sizes depending on how much coverage you want or need.
For example, a Twin comforter will fit on either a Twin or Twin XL, and a Queen comforter works well for both a full and queen-sized mattress.
With your duvet cover, it's possible to size up as well. If you have a queen-sized duvet insert, you could purchase an additional king-sized cover. This way, you can switch up the style, size, or pattern with different covers while still keeping the inner duvet in place.