Can I put just one size of a product on sale?

Yes. Every variant has its own sale price.

1. Open the product and go to the Variants card.
2. Find the variant row, for example "S".
3. Type the discounted price in its Sale column. Keep the normal price in Price.
4. Click Save.

On the product page, the sale price applies only when the shopper picks that variant. The other sizes keep their normal price. Product cards show the lowest variant price, so the discounted price can also appear on the card.

To end the sale, clear the Sale box for that row and click Save again.