Multi-region size matrix (US/UK/EU/CM/JP), how do you architect it on Magento?

Footwear is the only category where size is the primary failure mode. The architecture I ship:

• One canonical size attribute per gender segment, size_us_mens, size_us_womens, size_us_kids as the source of truth (US is the most-used in the data).
• Conversion attributes auto-populated, size_uk, size_eu, size_cm, size_jp derived from US via a conversion table stored as Magento attribute options (per gender, since men’s and women’s convert differently). Auto-update on configurable-product save.
• PDP swatch shows all five, customer sees “US 9 / UK 8.5 / EU 42.5 / CM 27 / JP 27” on hover. The default-displayed size matches the customer’s geo (US store view shows US, EU view shows EU).
• Cart + order shows all five, on the line item, in the email confirmation, on the invoice. International customers get the size they ordered in their own units even if shopping a US store view.

The Hoka / Brooks / Allbirds-specific fit-notes (e.g. “Hoka runs 0.5 large”) live as a custom PDP widget that pulls from a brand attribute. Fit-guide overlay opens on click and pulls last-bought size from customer history if logged in. This is the architecture that drops size-related returns ~25% without AR.