Obviously for as long as people have been wearing footwear, it has come in various sizes. But how long ago were standardized sizes established? When could you go into a cobbler’s shop and tell them you want particular footwear in a particular size?
Yes, there is an American system and European system for sizing. I don’t know how long the American measurement system has been in place. But much like women’s dress sizes, the application has changed through the years.
I used to wear a 9.5 American running shoe decades ago. Then the manufacture turned to Taiwan and ultimately China. I suppose in an attempt to charge more for making bigger sizes, the marked size began to change. I got up to buying size 11 and 11.5 at times. The Chinese have seen the light and now they are more accurately marked for sale in the US market.
Nothing beats actually measuring the foot at the time of shoe selection and of course trying the shoe on and testing it. This supersedes all measurement schemes.
I bought a new pair of trainers yesterday and there were no less that four different sizing systems noted on the box.
In the UK, we’ve had the same shoe size system since 1324: