Add custom field automatically (add_post_meta) with value based on number of words of article

Hook into save_post, count the words, and update the post meta field.

Sample code, not tested:

add_action( 'save_post', 'wpse_73563_save_word_count', 10, 2 );

function wpse_73563_save_word_count( $post_ID, $post )
{
    if ( ! current_user_can( 'edit_' . $_POST['post_type'], $post_ID ) )
    {
        return FALSE;
    }

    $count = t5_word_count( $post->post_content );

    update_post_meta( $post_ID, '_word_count', ( 200 > $count ? 2 : 1 ) );
}