stage.addEventListener(Event.RESIZE, resizeHandler);
stage.dispatchEvent(new Event(Event.RESIZE)); // force stage resize event for normal cases 

function resizeHandler(event:Event):void {
  if (stage.stageHeight > 0 && stage.stageWidth > 0) {
    stage.removeEventListener(Event.RESIZE, resizeHandler); // only execute once
    // your initialization code here
  }
}

Fix Internet Explorer Stage Dimensions

ActionScript 3