<div onKeyUp="chklen()">      
$_MESSAGE$      
</div>      
Осталось символов: <input name="fcounter" type="text" class="codeButtons" size="6" value="20000" readonly="true">    

20000 можно заменить на свое значение...    

2) В самый верх шаблона устанавливаем:    

Code
<script language="JavaScript" type="text/javascript">      
function chklen()      
{      
max_limit=20000;      

str = document.addform.message.value;      
len = str.length;      

if (len > max_limit)      
{      
str = str.substring(0,max_limit);      
len = str.length;      
document.addform.message.value = str;      
}      

document.addform.fcounter.value = (max_limit - document.addform.message.value.length);      

while ( document.addform.fcounter.value<0)      
{      
document.addform.message.value = str.substring(0,len-1);      
str = document.addform.message.value;      
len = str.length;      
document.addform.fcounter.value = (max_limit - document.addform.message.value.length);      
}      
}      
</script>