Smart Export : add product post meta

// functions.php

function custom_product_meta($array)
{
        $array[] = '_length';
        $array[] = '_width';
        $array[] = '_height';

        //also works for size or color attributes, or data added by a plugin... to find the right key you need :

        return $array;
}
add_filter('wcse_included_order_product_keys_filter', 'custom_product_meta');