prova questo

<script language="Javascript">
var x = Math.round((screen.width/2)-(290));
var y=((screen.height/2)-(205));
newwindow=window.open('tua pagina.html', 'titolo', 'width=580,height=410,screenX=' + x + ',left=' + x + ',screenY=' + y + ',top=' + y + ',scrollbars=no')
window.opener=self
</script>