Basta usare Js:

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script language="JavaScript">
<!--
function pop(url,name,height,width){
		     newpop=window.open(url,name,'toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=no,resizable=no,width='+width+',height='+height);
           }
//-->
</script>
</head>

<body>
<form name="form1">
  <select name="menu1" onChange="pop('new.php','prova',500,500);">
    <option selected>unnamed1</option>
    <option>unnamed2</option>
    <option>unnamed3</option>
    <option>unnamed4</option>
  </select>
</form>
</body>
</html>
Questo è tutto.Alla selezione immediatamente si aprirà una popup.
Ciao