Probá con éste codigo
<html>
<head>
<title>Go Fullscreen</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script type="text/javascript" src="js/swfobject.js"></script>
<script type="text/javascript">
var fo = new Object();
function initFlash()
{
fo.flashvars = {};
fo.params = {
allowScriptAccess: "always",
allowFullScreen: "true"
};
fo.attributes = {};
swfobject.embedSWF("swf/FlashFile.swf", "flashcontent", "100%", "100%", "10", "swf/expressInstall.swf", fo.flashvars, fo.params, fo.attributes);
}
</script>
</head>
<body>
<div id="flashcontent"></div>
<script type="text/javascript">
initFlash();
</script>
</body>
</html>
Sino hay una orden :
Stage.displayState = "fullScreen";
Saludos