Вы можете установить свойства ширины и высоты NativeWindow в Capabilities.screenResolutionX и Capabilities.screenResolutionY
public function AppWindow()
{
var options:NativeWindowInitOptions = new NativeWindowInitOptions();
options.systemChrome = NativeWindowSystemChrome.STANDARD;
options.type = NativeWindowType.NORMAL;
super(options);
width = Capabilities.screenResolutionX;
height = Capabilities.screenResolutionY;
// to make sure content doesn't scale and aligns to top left set stage properties
stage.align = StageAlign.TOP_LEFT;
stage.scaleMode = StageScaleMode.NO_SCALE;
}