In the early 1300s, Britain established the first means to measure shoe size. King Edward II declared barleycorn as the basis for shoe measurement. He ruled that the length of three barleycorns were equivalent to one inch, making this the standard for sizing.
There are several different shoe-size systems that are used worldwide. These systems differ in what they measure, what unit of measurement they use, and where the size 0 (or 1) is positioned. Only a few systems also take the width of the feet into account. – Wikipedia