a.htm
<script language="javascript">
var W = 200,H = 200;
var Top = (window.screen.availHeight-H)/2, Left = (window.screen.availWidth-W)/2;
window.resizeTo(W,H);
window.moveTo(Left,Top);
</script>
b.htm
<script language="javascript">
var W = 300,H = 300;
var Top = (window.screen.availHeight-H)/2, Left = (window.screen.availWidth-W)/2;
window.resizeTo(W,H);
window.moveTo(Left,Top);
</script>