Both full beds and twin beds are 75 inches (191 cm) long. They differ when it comes to width, however. At 54 inches (137 cm) wide, a full size bed is nearly as wide as a queen size bed. Twin beds are much narrower, measuring a mere 39 inches (99 cm) across. Non-Standard Dimensions. Both full and twin beds have extra-long, or XL, versions, which add an extra five inches (13 cm) to the bed's length. At 80 inches (203 cm), this makes these versions of full and twin size beds as long as king and queen size beds.