Show A Modal Window In A Frame Based Page?

I am trying to show a modal window in a frame based page.I am creating the modal window with div tags dynamically.I am using below js code to set css properties of the div as below:

Code:

function setModalWinStyle(frame){
frame.document.getElementById('modalLogin').style.position="fixed";
//frame.document.getElementById('modalLogin').style.position="absolute";

[code]...

This gives me result of a fixed modal window in the center (but only looks like in center), relative to my Content Frame (which is the middle frame with other frames as Header, Nav and Footer frames).How can I get the width and height of the viewport for the whole page at a time to center the modal window relative to actual center of the page not just content frame. Also I need to calculate the resize factors, to make it center when window resizes.