First of all I created a Woocommerce product with multiple variations according to two attributes, then I exported it. This allowed me to see the structure of the import file.
After that I created my product database outside WordPress, respecting the structure required. It’s about 220 products in total.
Before importing the new database I deleted everything that existed: products, categories, attributes and media, just to be sure that everything was ok.
I imported the database and it looks like everything is right, I can see all my variations but each one has attribute 1 (size) with ‘Any Size’ and and attribute 2 (color) with ‘Any color’, so it looks like variations were not automatically associated to the right attributes, even thought that attributes have been correctly created in the ‘Attributes’ section.