Aizaz Imtiaz Awan
Unfortunately, the change that you are asking this time around is also the default functionality at the WooCommerce level which isn’t influenced by the Theme itself.
To add the Attributes under a Product whether it is Size or any other you eventually need to edit the Product first > and then define the Attributes and create the Variations for it. Thi is how it works under WooCommerce.
Although, you might know that you can Bulk Edit Products, if you want to assign some common things to all under the Dashboard > Products: https://snipboard.io/e2OdXp.jpg