The smallest size of mattress intended for an adult is the twin size mattress. The smallest mattress manufactured is the crib size mattress, meant to be placed in a baby's crib.
The standard size for a twin mattress is 38 to 39 inches in width and 75 inches in length. Twin mattresses come in the additional size twin XL. Twin XL mattresses have the same width as standard twin mattresses, but are approximately five inches longer. Many college dormitories use twin XL mattresses in their beds.
The standard size for a crib mattress is 27.5 inches in width and 52 inches in length.