Thumbnail image size/quality | WordPress.org

Hello,
Here is the hook for changing image size. You can pass width and height based on your needs.

// for thumbnail image size
function woocommerce_gallery_thumbnail_size_callback( $args ) {
  return array(240,240,0);
}
add_filter( 'woocommerce_gallery_thumbnail_size', 'woocommerce_gallery_thumbnail_size_callback', 99 );
// for main image  size
add_filter( 'woocommerce_get_image_size_single', 'woocommerce_get_image_size_single_callback',99);
function woocommerce_get_image_size_single_callback( $size ) {
  return array('width' => 800, 'height' => 600, 'crop' => 1, );
}

Thank You.