Customize WooCommerce thumbnail size

woo-thumb-size.php

<?php

add_filter( 'woocommerce_get_image_size_gallery_thumbnail', 'ci_theme_woo_gallery_thumb_size' );
function ci_theme_woo_gallery_thumb_size() {
  return array(
    'width' => 100,
    'height' => 0,
    'crop' => 0,
  );
}