ISO Standard Sizes
The ISO standard for shoe sizes is the mondopoint sizing system. This standard was first proposed in the early 1970's and eventually released in 1991. It has gained some traction, although it's use is far from widespread, with Mondopoint often being used for military footwear (adopted by NATO), ski boots and ice skates. Mondopoint is specified by ISO 9407-1991 - Shoe Sizes, Mondopoint System of Sizing & Marking. The document was reviewed and confirmed without change again in 2012.
ISO Standard Conversions
In 2015 the International Organisation for Standardisation (ISO) published ISO/TS 19407 - Footwear -- Sizing -- Conversion of sizing systems with standard conversion tables for shoe sizes and ISO/TS 19408 - Footwear -- Sizing -- Vocabulary and terminology giving background information on shoe sizes.