ScaleTransform scale = new ScaleTransform();

 public Window1()

 {

    InitializeComponent();

    this.Loaded += new RoutedEventHandler(Window1_Loaded);

 }

 void Window1_SizeChanged(object sender, SizeChangedEventArgs e)

 {

    ChangeSize(e.NewSize.Width, e.NewSize.Height);

 }

 void Window1_Loaded(object sender, RoutedEventArgs e)

 {

    orginalWidth = this.Width;

    originalHeight = this.Height;

    if (this.WindowState == WindowState.Maximized)

    {

        ChangeSize(this.ActualWidth, this.ActualHeight);

    }

    this.SizeChanged += new SizeChangedEventHandler(Window1_SizeChanged);

 }

 private void ChangeSize(double width, double height)

 {

    scale.ScaleX = width / orginalWidth;

    scale.ScaleY = height / originalHeight;

    FrameworkElement rootElement = this.Content as FrameworkElement;

    rootElement.LayoutTransform = scale;