Twin Beds have a mattress size of 39 in. x 75 in. Twin beds are generally used for children and come in many different styles, shapes and materials. When your child needs a new bed, make them feel special by letting them choose their own twin bed. Another popular use for twin beds is guest beds. Because of their small nature its easy to fit two twin beds into your guest room to accommodate for most guest needs, especially when your family comes for a visit.