add_filter( 'woocommerce_get_image_size_gallery_thumbnail', 'override_woocommerce_image_size_gallery_thumbnail' );
function override_woocommerce_image_size_gallery_thumbnail( $size ) {
    // Gallery thumbnails: proportional, max width 200px
    return array(
		'width'  => '170',
        'height' => 170,
        'crop'   => 0,
    );
}