A shoe size is an alphanumerical indication of the fitting size of a shoe

Shoe

A shoe is an item of footwear intended to protect and comfort the human foot while doing various activities. Shoes are also used as an item of decoration. The design of shoes has varied enormously through time and from culture to culture, with appearance originally being tied to function...

 for a person. Often it just consists of a number indicating the length because many shoemakers only provide a standard width for economic reasons.

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.