Each size and colour of a product is a separate product in WithWine. You then link them together with Product Variations, so they show together.
For example, a t-shirt in red, blue and green, each in small, medium and large, is 9 products.
Step 1: Create one product for each size and colour
Go to Settings > Products > Other Products and click + Add Product.
Create the first one, for example T-shirt Red Small, with its price, photo and settings. See How to add merchandise and other products.
To create the rest quickly, click Clone next to the product in the list, then change the name, photo and SKU code. See How to copy (clone) a product.
Step 2: Choose the default product
Choose one product to be the parent. The parent is the default: it's the one customers see first on your website, and they can pick the other sizes and colours from a dropdown. For example, make T-shirt Blue Large the parent.
Step 3: Link the other products to it
Go to Settings > Products > Other Products and click the parent product.
Click Product Variations. In the parent's Variation Type, type a name for it, for example Blue Large.
Click Add Product Variation.
Start typing the name of another size or colour and click it in the list.
In the variation type, type a name customers will understand, for example Blue Small. You can type your own name instead of choosing one from the list.
Leave Use Stock from Parent unticked, so each size and colour keeps its own stock.
Click Save on the row. Repeat for each size and colour (Blue Medium, Green Small, Green Medium, Green Large and so on).
Click Save at the bottom of the parent product.
The same approach works for gift cards in different values. See How to sell a wine by the glass or as a magnum (product variations) for more about how variations work.