Twin beds are 39 inches wide and 75 inches long, so a standard twin headboard is 42 inches wide. The twin extended headboard size is also 42 inches because the difference between the two beds is in the length. A twin extended bed is 80 inches long. The full-size bed, also known as the double bed, is 59 inches wide, not quite double that of a twin bed. The length is 75 inches.