A size bed is a standard mattress and foundation set that measures 54 inches wide by 75 inches long. This refers to both the dimensions of the actual mattress as well as the type of bed frame required, which typically has a headboard height of 54 inches and footboard height of 75 inches. A size bed is considered a full or double-sized bed depending on the width, with 54 inches being the standard for a full-size mattress and foundation set in North America.