Best way to add parent listings to a lot of Child ASINS

I have a bunch of child listings that need a parent.

Problem one is I don't know what is the best method for doing this. These involve one variation type and that is size.

These are auto decals.

When I downloaded a flat file(sticker decal) it does not have the option for size.

I found a video on youtube that mentioned using a category listings report as the file to join children to parents.

I have no parents currently. I don't need to make a sku for the parent but it seems with a flat file you must for it to know which children to associate to which parent when doing a lot.

What is my best method here? The category listings report seems nice because it has all of the content already there vs exporting out the asins and skus and adding them in manually for hundreds of listings. I am trying to make this an easy process. Up until now it has been done in seller central and that is slow when you need to update or join 300+ listings.