size

Size is the most common filter applied in apparel Shopping, ahead of even colour and price. Items without size fall out of size-filtered searches, and apparel-specific Shopping placements often require size to be populated at all. For non-apparel products where dimensions matter (furniture, electronics, packaging), size carries weight too but with looser filtering.

The regional sizing problem is where most catalogs need `size_system` paired with the size value. UK 12, US 12, EU 12, and JP 12 are four different physical sizes. Without `size_system` declaring the standard, channels guess based on locale and the guesses are often wrong. Multi-region catalogs serving UK and US markets especially need explicit size_system declarations — otherwise UK 12 dresses get shown to US users expecting US 12 (much smaller) and the returns flood in.

Variant grouping under `item_group_id` requires size to disambiguate variants. Without size populated on each variant, channels can't tell which variant won the click, which kills variant-level performance reporting and breaks size-specific retargeting.