Add Selectable Options (Like Size or Color) to a Product
To add selectable options, go to Categories("yourdomain.com/categories"). There, either add a new category or edit the existing one. Set its "Seller listing type" to "Sell Something".
You can now add the additional fields such as size, color, price of each color, etc. for this listing. Fill up those fields and post the listing.
Now, your listing will look like this:
Your customers can now choose the selectable size, color, shipping type, etc.