Filtering by Size

[PupaBoutique]

Hi - I'm currently using free theme Supply and i would like to add filtering by size on the left hand side of the page, instead of the default option of collection's name (which is completely impractical and negative). I'm surprised Shopify offer such average themes (in addition, they have removed the 24/7 chat option from Shopify support center).

Can anyone please advise how to add Filtering by Size inside of each collection? Does anybody have the code to share and the location to put it? Thanks a lot!