Mattress sizes are fairly standardized across all manufacturers, but there is often confusion when it comes to the difference between a twin and a twin XL. A twin mattress measures 38 inches wide by 75 inches long. A twin XL is a bit longer, measuring 38 inches wide by 80 inches long. While a twin mattress may be slightly less expensive than a twin XL, it can feel cramped for those who like to stretch out on their mattress or anyone over 6 feet tall.