If you use WooCommerce 3.3.x 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.
To change product widget image size go to Settings > Media > Thumbnails Size.
More information about product image settings you can find in WooCommerce Documentation.
After these changes, you need to Regenerate Thumbnails to apply them! You can use Regenerate thumbnails plugin to do this.