What size is a double flat sheet? Rebecca Leonard February 13, 2020 10:56 Updated Follow Our double flat sheets measure 228cm x 254cm. Shop double flat sheets. Related articles What size is a double duvet cover? What size is a double fitted sheet? What is a deep fitted sheet? What size is a King flat sheet? What materials are used to make our Cotton bed linen and what are their benefits? Comments 0 comments Please sign in to leave a comment.