A Twin XL mattress is 38 inches wide and 80 inches long. This size is 5 inches longer than a standard Twin mattress, which is 38 inches wide and 75 inches long. The Twin XL size is best suited for single sleepers who are over 6 feet tall or growing children and teens that may exceed 6 feet eventually. The Twin size is best suited for single sleepers who are under 6 feet tall. Both bed sizes are only suited for single sleepers and are not recommended for couples.