Simply copying and pasting the snippet to your theme’s functions.php file and you will be able to define a minimum word count per post in WordPress.
$num = 100; //set this to the minimum number of words
$content = $post->post_content;
if (str_word_count($content) < $num)
wp_die( __('Error: your post is below the minimum word count.') );
Snippet Source/Credit: Pippin Williamson