google apps script to archive and autoreply to long emails.

filter_long_emails.js

/* paste into http://script.google.com
   run periodically (e.g. every 5 minutes) with a timer by selecting 'Resources' > 'current project's triggers...'
*/
function filterLongEmails() {
  var num_messages = 1;       // will be applied to the last n messages. Google allows up to 500
  var word_limit = 50;
  
  var subj = "Shorter emails will get read.";
  var body = "Dear friends, I value my time and yours but I appreciate it if you can keep your emails under " + word_limit + " words. Please edit and resend.";
  
  // get the last 'num_messages' from inbox
  var threads = GmailApp.getInboxThreads(0, num_messages);
  for (var i = 0; i < threads.length; i++) {
    if (threads[i].getMessageCount() == 1) {
      var msg = threads[i].getMessages()[0];
      var word_count = msg.getBody().split(' ').length;

      if (word_count > word_limit) {
        MailApp.sendEmail(msg.getFrom(), subj, body);
        GmailApp.moveThreadToArchive(threads[i]);
      }
    }    
  }
};