A: A Twin extra long is known most commonly as a Twin XL. The XL stands for extra long. The dimensions of a extra long twin beds are 39 inches wide by 80 inches long. This makes a twin extra long mattress measure 5 inches longer than a standard twin size mattress. Shop bedding for twin extra long bed. A: Twin Extra Long is the bedding and sheet size for extra long twin beds. Twin Extra Long bedding is required in most American college dorm rooms. This size is also popularly referred to as college bedding or dorm bedding. Extra long beds can be hard to shop for.