Not recommended because what will happen is the browser window will resize and this pisses off everyone that uses the net. An alternative is to leave the index page and have java script which opens up a new window in your specified size in front of the browser window that causes the popup. This is what many sites use that try and do something similar to what you are looking for.
Taking control of browser windows and sizes away from the user is bad usability.
In saying that, here is the code you should put in your page that resizes when loaded.
Quote:
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<body onload="window.resizeTo(400,400)">
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replace the numbers with the width and height you require. It works in both latest versions of IE and Firefox.