Duvet Cover Too Small . A full casper mattress is 53” x 75”, and requires full flat sheets that are 96″ x 102″. Sijo french linen duvet cover. WrinkleResistant Comforter Cover, Floral. To go on our fullsize duvet from www.pinterest.com Second, it will depend on the material and the way your insert is made. Make sure the duvet insert lines up evenly with all the edges and corners of the cover. Of course that means i'm in a decorating frenzy.