Twin XL Mattress Size is 39 inches wide x 80 inches long. These are standard twin XL bed measurements. Twin XL Mattress depths can vary from just a few inches to 16" or more for the new huge pillow tops. Many times, actual twin XL mattress dimensions will vary by one or two inches, usually smaller. Twin XL Size Bedding used to be mainly used in college dorms. However, with our population growing taller, many children cannot fit in traditional twin size beds so twin XL is becoming much more popular. I would not be surprised to see twin XL mattresses to begin to threaten sales of twin size beds in the future.