A twin size mattress is narrower than a full size mattress, according to Consumer Reports. A twin or single mattress is 38 inches wide by 75 inches long. A full or double mattress is 53 inches wide by 75 inches long, which is 15 inches wider than a twin.
A full mattress provides each partner with 27 inches of personal sleeping space, which is about the width of a crib and much less room than a single person would have sleeping in a twin bed, says The Better Sleep Council. Both twin and full size mattresses may be too short for some people, especially adult men. They are not recommended for teens and adults over 5 feet 5 inches tall. Taller individuals might consider a twin extra-long (XL), which is 5 inches longer than a regular twin.