An extra long twin mattress is 39 inches in width and 80 inches in length. A traditional twin mattress is five inches shorter at 75 inches in length.
An extra long twin mattress is often used in dorm rooms, and makes a nice option for guest rooms since two extra long twin mattresses pushed together makes the same dimensions as a king size bed. This versatility allows you to separate for children or push together for adults or couples that may be using the guest room. Purchasing bedding may become an issue, since most bedding is made to the traditional twin mattress dimensions.