What size is a double flat sheet? Rebecca L April 10, 2020 12:05 Updated Follow Our double flat sheets measure 228cm x 254cm. Shop double flat sheets. Related articles What size is a King flat sheet? What size is a double fitted sheet? What size is a double duvet cover? What is a deep fitted 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.