Limit number of words in wordpress post

functions.php

<?php
// put this is functions.php

define('MAX_POST_WORD_COUNT', 200); // change this to set the maximum word count

add_action('save_post', 'limit_post_word_count'); // run the check whenever a post is created or saved using any method

function limit_post_word_count ($post)
{
  if (str_word_count($post->post_content) > MAX_POST_WORD_COUNT)
  {
    wp_die('Your post is too long, it must be less than ' . MAX_POST_WORD_COUNT . ' words long.');
  }
}