Edit because i said some wrong things :
So here is how to get a message from the ckeditor, convert it from html to bbcode, and count caracters in the message without bbcode :
PHP Code:
require_once(DIR . '/includes/class_wysiwygparser.php');
$html_parser = new vB_WysiwygHtmlParser($vbulletin);
$contenu = $html_parser->parse_wysiwyg_html_to_bbcode($vbulletin->GPC['message'], FALSE);
require_once('includes/class_bbcode_alt.php');
$bbcodeparser_plain = new vB_BbCodeParser_PlainText($vbulletin, fetch_tag_list());
$message_plain = $bbcodeparser_plain->do_parse($contenu);
$count = vbstrlen($message_plain);
if($count > $maximum){
//Is too big
}