A standard single or twin mattress is 39 by 75 inches. Twin beds are convenient for children's rooms or for rooms used for multiple purposes but may be too short for a tall adult. Some manufacturers make an extra-long twin mattress that measures 39 by 80 inches to accommodate adults.
Standard twin beds use fitted sheets that measure 39 by 75 inches and flat sheets that measure 66 by 96 inches. Twin sheets are easy to find in a wide selection of colors and patterns and are less expensive than sheets sized for larger beds. Extra-long twin mattresses use fitted sheets that are 39 by 80 inches and flat sheets that are 66 by 102 inches, but these sizes are not as easy to find.