A king platform bed frame should be at least the measurements of the king size mattress, which is 76 inches by 80 inches. A platform bed frame is easily built by a do-it-yourselfer, to save money and create an off the floor platform for your mattress. If you have a platform, you won't need a box spring for your bed. Some people like the platform to be larger than the mattress, so you should add the amount to both the width and length that you wish to increase the size. Take into consideration the bulk of any mattress pad and bedding you will use also.