What size images does the plugin use?

I want to delete all media files not used and in future media settings only set sizes that I need. Does the plugin get thumbnails from the resized images in the media settings or does it resize images by itself?

Does the plugin require specific image sizes to be defined in the media settings? I want to ensure that images are accessible in the correct size and not resized by the browser since this will cause the site to be slower. I am referring to the thumbnails since full-sized files can vary per product.

Thanks in advance.