<div onKeyUp="chklen()">
$_MESSAGE$
</div>
Осталось символов: <input name="fcounter" type="text" class="codeButtons" size="6" value="40000" readonly="true">

<script language="JavaScript" type="text/javascript">
function chklen()
{
max_limit=40000;

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>