Customizer → Products → click a product → Sizes tab.
Reusable size runs (Adult S–3XL, Youth, EU numeric…) are defined once in Assets → Sizes:
Open SizesNote
The Sizes tab is only shown when Multi-size export is enabled in Settings → Features. Without it, products use a single pattern and the tab is hidden.

Each product carries its own list of sizes. Per size you can attach:
S, M, 2XL, 38…). Codes must be unique within the product..svg.chest, length) and a label, and type the real value in cm or mm.Toggle Show size chart in customizer. Only key, label and the value are published — the pattern geometry never leaves the dashboard.
Which measurement does the print service use for scale? width first, then height, then the first measurement on the panel. Make sure at least one of those exists on every size, then regenerate print files.
My customer picked a size that has no pattern. The quote is accepted, but Production cannot export that size until a pattern is uploaded. The product list shows a muted scissors icon on sizes without patterns.
Sizes tab is missing. Enable Multi-size export in Settings → Features.