A product has its own size chart and its category has another one. Which one do shoppers see?

The chart assigned to the product wins. When a shopper opens a product page, Wcart first looks for an active chart with that product in Assign to product IDs. Only if there isn't one does it use a chart assigned to one of the product's categories.

So for your slim-fit shirts:

1. In Plugins → Size Charts, keep the general chart on the Shirts category in Assign to category IDs.
2. Create a second chart and put the slim-fit product IDs in Assign to product IDs.

If two charts match the same way, the most recently updated one is shown.