prova questo
ma se il computer esplode non chiedermi i danni

codice:
<script language="JavaScript1.2" type="text/javascript">
var screenWidth=screen.availWidth;
var screenHeight=screen.availHeight;

var posx = (screenWidth-660)/2
var posy = (screenHeight-460)/2

if (screenWidth > 800)
{

window.moveTo (posx,posy)
window.resizeTo(660, 460)
}


</script>