WP_Query() order by post content lenght?

How do I order a post query by the length of the post content?

$orderby = ?;

$query = array(
  'posts_per_page' => 10,
  'post_status' => 'publish',
  'orderby' => $orderby,
  'order' => 'DESC',
); 
$the_query = new WP_Query($query);