Best For: A full bed is as long as a twin bed at 75 inches but much wider at 54 inches and can fit an average-size adult. Full-size mattresses are great for growing children, single sleepers, and rooms with narrower floor space. Full-size mattresses measure 54” X 74” and fit easily. Full Mattress Size Dimensions: 54” x 75”. A Full-size mattress is 54 inches wide and 75 inches long ( cm x cm). Also known as double beds.