add_filter( 'preprocess_comment', 'wpb_preprocess_comment' );
function wpb_preprocess_comment($comment) {
    if ( strlen( $comment['comment_content'] ) > 5000 ) {
        wp_die('Comment is too long. Please keep your comment under 5000 characters.');
    }
if ( strlen( $comment['comment_content'] ) < 60 ) {
        wp_die('Comment is too short. Please use at least 60 characters.');
    }
    return $comment;
}