Fijate como es que lo tengo yo en mi sitio, a mi me funciona muy bien ...
 
<title>Tu Pagina</title>
<SCRIPT languaje="JavaScript">
<!-- Begin ACA COMIENZA EL ANIMADOR DE VENTANAS 
//Animated Window- By Rizwan Chand (
[email protected])
//Modified by DD for NS compatibility
//Visit 
http://www.dynamicdrive.com for this script 
function expandingWindow(website)
{
  var windowprops='width=1,height=1,scrollbars=yes,statu  s=yes,resizable=yes'
  var heightspeed = 6; // vertical scrolling speed (higher = slower)
  var widthspeed = 14;  // horizontal scrolling speed (higher = slower)
  var leftdist = 0;    // distance to left edge of window
  var topdist = 0;     // distance to top edge of window 
  if (window.resizeTo && navigator.userAgent.indexOf("Opera") == -1)
  {
    var winwidth = window.screen.availWidth - leftdist;
    var winheight = window.screen.availHeight - topdist;
    var sizer = window.open("/Espere.asp","ventana","left=" + leftdist + ",top=" + topdist +","+ windowprops); 
  for (sizeheight = 1; sizeheight < winheight; sizeheight += heightspeed)
    sizer.resizeTo("1", sizeheight); 
  for (sizewidth = 1; sizewidth < winwidth; sizewidth += widthspeed)
    sizer.resizeTo(sizewidth, sizeheight); 
  sizer.location = website;
  }
  else
    window.open(website,'ventana');
} 
//  End -->
</Script>
</head>
<body> 
Eso en el head 
y despues ... 
<form action="/Compras.asp" method="post" target="ventana" onsubmit="expandingWindow('/Compras.asp')">
<input name="_IdUsuario" type="hidden" value="NroUsuario">
<input type="submit" value="Mi Carrito" name="Mi Carrito">
</form> 
Estudialo y fijate como lo adaptas ...