Since a duvet is a loose piece of bedding, nothing is stopping you from using a size larger or smaller than your mattress. Using a larger size gives you more overhang and can ensure full coverage if you buy a duvet with a high loft. A smaller size can be good if you’re just looking for a way to keep warm, are on a tight budget, or share a bed with a hot sleeper who doesn’t need the warmth of a duvet.