The size of the platform bed varies from one brand to another. Many brands feature all four sizes, namely king, queen, full, and twin size. However, not all brands selling platform beds will have all the aforementioned sizes. In case you want a specific size of the bed, many stores will happily provide customized sizes. By general standards, the king size is somewhere around 76- by 80-inches, a queen measures 60-by 80 –inches. The full sizes are 54- by 75-inches and twin being the smallest size available is 39- by 75-inches.