[N!ck]

you could use a javascript, like this in the head:

<script language="JavaScript">
function unlength (form) {
 if (form.blah.length > 15) {
  alert("Your username is too long!");
  form.blah.focus();
 }
}
</script>

and then

<input type="text" name="blah" onBlur="unlength(this.form)">

of course, i'm no expert @ javascript; this COULD have an error