<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>