Maybe a feature for future versions

Hi,
I have no problems with the plugin, it’s working as expected.
I would like to propose additional option, to choose an image size (from the array of registered image sizes) if the option is featured image.
It’s just a matter of adding and option to admin page and replacing wp_get_attachment_url($post->ID) with wp_get_attachment_image_src( $post->ID, $selected_image_size)[0]
The problem is with portrait images, they get pulled correctly, but facebook crops them at the middle, leaving headless image on the post. Another problem would be huge images that people upload, so facebook won’t fetch the image properly every time.

I’m a developer and I am capable of doing this myself, but the changes would be erased on plugin update.
Hope You’ll consider this.
Thanks.