Hmm.... I guess if you wanted you could use the javascript to dynamically write part of the DOM so that a new <link> to a stylesheet is created. Link to style.php?size=WxH and then style.php would take those values, include whatever CSS, then send back the Content-Type header with "text/css" and you've got your stylesheet.
Alternatively, you could send an AJAX request, or use a redirect page and set the value in a cookie. I'm not too sure I'd want to visit your site because as I browse, and my screen realestate needs change, I modify my browser window. If your script doesn't take that into account, you could have major issues.
