Change WooCommerce product gallery image thumbnail dimensions

woocommerce-product-gallery-thumbnails-dimensions.php

<?php

/**
 * Change WooCommerce product gallery image thumbnail dimensions.
 *
 * @return Array  Array( height, width )
 */
function your_prefix_gallery_image_sizes() {
    return array(
        600,
        600
    );
}

add_filter( 'woocommerce_gallery_thumbnail_size', 'your_prefix_gallery_image_sizes' );