The size of a single duvet can vary slightly, but the standard size for a single duvet is 135 x 200 cm or 53 x 79 inches. This size is designed to fit a single bed, which typically measures 90 x 190 cm or 36 x 75 inches. Its important to note that there are also long single duvets, which are 135 x 220 cm or 53 x 86 inches, and are popular in Europe and at IKEA in the UK. These variations in size provide options for individuals who may prefer a longer or wider duvet for their single bed.