Whether you use A4 paper in your typewriter, or - as God intended - quarto, the question remains: why are paper sizes... well this size? The answer to this (well in the case of US 8.5 x 11 inch paper anyway) lies in the history of how paper was made. In the early days of paper making it was largely done by hand. Workers dipped paper mold frames into vats of wood pulp. Once dried, the wood pulp in the frames was now a sheet of paper. It was found that frames which were forty-four inches wide were best for the workers to handle with their outstretched arms.
Each 44 inch sheet was folded into four to give four eleven inch wide sheets of paper. Naturally use of the typewriter and later printing technologies helped to standardise paper sizes but basically it is largely down to how long people's arms are!
