ti posto un esempio che ho salvato nell' hdd
codice:
<script type="text/javascript"> 
function textCounter(field, countfield, maxlimit) { 
  if (field.value.length > maxlimit) 
    field.value = field.value.substring(0, maxlimit);
  else 
    countfield.value = maxlimit - field.value.length; 
}
</script>
<form> 
( Puoi mettere 125 caratteri. )

<textarea name=message wrap=physical cols=28 rows=4 onKeyDown="textCounter(this.form.message,this.form.remLen,125);" onKeyUp="textCounter(this.form.message,this.form.remLen,125);"></textarea> 

 
<input disabled="disabled" type="text" name="remLen" size="3" value="125">caratteri rimanenti
</form>
ciao