Hi,
You can use the "export" button of the products listing on your website. That will give you a CSV file with the first lines being for regenerating your categories tree, and then the products and variants data. So you can study the CSV file with excel.
The text :
product_code,product_parent_id,size,price_value
main_product,,big,
first_variant,main_product,big,15
second_variant,main_product,small,10
is actually the content of the CSV file.
Each column is separated by a comma. And on the first line, you have the name of the columns.
If you save that text in an empty txt file and rename it to mycsv.csv you can then open it with Excel (or even better, LibreOffice's Calc) and you'll see the columns and the rows.