You'll want to identify the element or container ID and do something like this:
document.getElementById('MyModal').style.height = '500px';
document.getElementById('MyModal').style.width = '800px';
If you are using jQuery for this it can be quite a bit easier as you can attach it to the actual show modal function.
Edit
Within the javascript functions above, MyModal will be the ID of the container or modal. For example, if you are using a DIV for the outer element of your modal, you would set the DIV up like this:
<div id='MyModal' class="IFNEEDED">CONTENTS OF MODEL</div>
EDIT #2
Since this is not a "modal" as most would describe today, its more of a new window popup, this line of code should work for you (found it here):
window.open('http://www.pageresource.com/jscript/jex5.htm','mywindow','width=400,height=200')