I just added the following lines before DB_save in the story.php.
Text Formatted Code
$word_length= 470;
$strlen_bf = mb_strlen($introtext,'utf-8');
$introtext = mb_strimlen($introtext,0,$word_length,"...",'utf-8');
$strlen_af = mb_strlen($introtext,'utf-8');
if($strlen_bf <> $strlen_af){
$display .= COM_siteHeader('menu', $LANG24[5]);
$display .= COM_errorLog('Your intro text is too long to submit.', 2);
$display .= storyeditor($sid);
$display .= COM_siteFooter();
echo $display;
exit;
}
You could define the word_length as $_CONF['word_length'] in custom config.file.