In North American sizes, a king-size duvet varies, and it can be as small as 90 by 86 inches or as large as 90 by 102 inches. A duvet of this size is meant to fit a king-sized mattress, which is typically 76 by 80 inches.
Depending on the manufacturer, duvet covers and sheets vary slightly. A fitted sheet for a king-sized mattress is approximately 76 by 80 by 8 inches, with a flat sheet being 110 by 114 inches. A comforter for a king-sized mattress may be as large as 112 by 106 inches. A duvet cover for a California king mattress is approximately 94 by 104 inches.