If you use WooCommerce 3.3.x or higher, then use WordPress Customizer (it’s a feature in the WordPress admin) to change the catalog and single product Image settings.

Go to Appearance > Customize > WooCommerce > Product Images.

More information about product image settings you can find in WooCommerce Documentation.

8theme product widgets and gallery images use WP thumbnails size (Settings > Media).

Note:

After these changes, you need to Regenerate Thumbnails to apply them! You can use Regenerate thumbnails plugin to do this.

Note:

We would recommend to use a graphics editor and prepare product images (make them the same size) before uploading it to your media gallery, then your grid will look much better than if you use crop option.

Note:

Do not forget to optimize images before uploading on site. You can use https://tinypng.com/ for this.