Metatags trimmed body in description/summary
You can use any field with Metatag, so simplest way would be to use Computed Field, that


  lets you add a custom "computed fields" to your content types. These computed fields are populated with values that you define via PHP code. You may draw on anything available to Drupal, including other fields


In your custom php code for field computation, use code like this:

$body = $entity-&gt;body['und'][0]['value'];
$width = 160;
$body_trimmed = substr($body, 0, strpos(wordwrap($body, $width), "\n"));
$entity_field[0]['value'] = $body_trimmed;


It will truncate on the word boundary, giving you no more than $width characters. More about truncating on StackOverflow.

Then, simply feed this computed field to Metatag.