Всё, ОК.

Код AS1/AS2:
Stage.align = "TL";
Stage.scaleMode = "noScale";
var stageListener:Object = {
onResize:resizeHandler
};
Stage.addListener(stageListener);
function resizeHandler():Void{
var w:Number = Stage.width;
//var h:Number = Stage.height;
trace(w);
};
resizeHandler();