In the dialog box with the modal option set to true, the following div is added where the width and height represent the viewport in the browser window:
<div class="ui-widget-overlay" style="width: 607px; height: 350px; z-index: 1001;"></div>
You can try something similar on your page. See http://jqueryui.com/dialog/#modal and then browse the source in the iframe demo for the full source.
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