Combine product sizes

/**
 * Get products sizes
 */
function bis_get_sizes($continue_import, $post_id, $data, $import_id) {
    // Get the product ID & size
    $product_id = $data['manufacturer_artno'];
    $product_size = $data['size'];

    $sizes = (array) get_transient('bis_sizes');
    $sizes[$product_id][] = $product_size;
    set_transient('bis_sizes', $sizes);

    return $continue_import;
}
add_filter('wp_all_import_is_post_to_update', 'bis_get_sizes', 10, 4);

/**
 * Reset transient
 */
function bis_reset_sizes() {
    set_transient('bis_sizes', '', 60*60);
}
add_action('pmxi_before_xml_import', 'bis_reset_sizes');

/**
 * Save sizes as product attributes
 */
function bis_save_sizes() {
    $all_sizes = (array) get_transient('bis_sizes');
    $taxonomy = 'pa_size';

    // Remove empty items
    $all_sizes = array_filter($all_sizes);

    if(!empty($all_sizes)) {
        foreach($all_sizes as $sku => $product_sizes) {
            $product_id = wc_get_product_id_by_sku($sku);
            $product_terms = array();

            // Add product sizes again
            foreach($product_sizes as $product_size) {
                $term = term_exists((string) $product_size, $taxonomy);

                if(empty($term)) {
                    $term = wp_insert_term((string) $product_size, $taxonomy);
                }

                $product_terms[] = (int) $term['term_id'];
            }

            wp_set_post_terms($product_id, $product_terms, $taxonomy);
        }
    }
}
add_action('after_xml_import', 'bis_save_sizes');

add_action('init', function() {
    if(isset($_GET['test'])) {
        bis_save_sizes();
        die();
    }
});