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

<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>