Daybed mattress sizes are usually twin size bed dimensions of 39 inches by 75 inches. There is a full or double daybed size of 54 inches by 75 inches but these are a bit rare. These are Daybed size mattress measurements. Daybed Size Mattress depths can vary from just a few inches to 14" or more. Generally, you want to keep a daybed mattress lower profile or lower height so you don't hide the fancy backboard. Many times, actual daybed mattress dimensions will vary by one or two inches, usually smaller.