Something

private function onResizeHandler(){
	var diffWidth:Number = Stage.width - this._width;
	var diffHeight:Number = Stage.height - this._height;
	var aRatio:Number = this._width / this._height;
	if(diffWidth >= diffHeight){
		this._width = Stage.width;
		this._height = Stage.width / aRatio;		
	} else {
		this._height = Stage.height;
		this._width = Stage.height * aRatio;
	}
}