A single, or twin, mattress measures about 39 inches by 75 inches. An extra-long twin bed is 80 inches long and may be more suitable for an adult. Single bunk beds measure 39 inches by 75 inches, but narrow bunks measuring 36 inches wide are also available.
A single bed is the smallest standard bed size. Only a crib mattress is smaller. A single is usually recommended for children and teens or for use in a guest room or college dorm. Sizes may vary slightly from one manufacturer to another.
The next bed size up is a double, or full. Double beds measure about 54 inches by 75 inches and can hold two adults. However, larger adults may be more comfortable with a queen bed, which measures 60 inches in width.