@ism-freelance I answered that above:
“you need to define these as product options on the relevant product and use product options in the add to cart API. Also, you will want to update the displayed price when selecting the size via whatever custom control you might be using. You can choose to hardcode the price in your code, or use https://www.wix.com/code/reference/wix-stores-backend.html#getProductOptionsAvailability in order to get the price for the selected variant”
If this isn’t clear or you require more info please let me know