Change the thumbnail size of the WooCommerce product gallery

woocommerce-product-gallery-thumbnails-dimensions.php

<?php

/**
 * Change the product gallery thumbnail size.
 *
 * @param Array $size
 * @return Array
 */
function your_prefix_modify_gallery_thumbnail_size( $size ) {
  
	return array(
		'width'  => 100,
		'height' => 100,
		'crop'   => 0,
	);
  
}

add_filter( 'woocommerce_get_image_size_gallery_thumbnail', 'your_prefix_modify_gallery_thumbnail_size' );