How to adjust the length of [node:summary] for metatags?
You can do a few different things.
Assuming you are using the metatag module.

1) Prevent metatags from saving with more than 150 chars using a presave hook:

function hook_metatag_presave(&amp;$metatags, $entity_type, $entity_id, $revision_id, $langcode) {
  dsm($metatags);//debug what the array is. You can trim the values for the metatags here.
}


2) Alter the metatag before bing cached:

/**
 * Alter metatags before being cached.
 *
 * This hook is invoked prior to the meta tags for a given page are cached.
 *
 * @param array $output
 *   All of the meta tags to be output for this page in their raw format. This
 *   is a heavily nested array.
 * @param string $instance
 *   An identifier for the current page's page type, typically a combination
 *   of the entity name and bundle name, e.g. "node:story".
 * @param array $options
 *   All of the options used to generate the meta tags.
 */
function hook_metatag_metatags_view_alter(&amp;$output, $instance, $options) {
  if (isset($output['description']['#attached']['drupal_add_html_head'][0][0]['#value'])) {
    $output['description']['#attached']['drupal_add_html_head'][0][0]['#value'] = truncate_utf8($output['description']['#attached']['drupal_add_html_head'][0][0]['#value'],150);
  }
}