I suppose the ActualHeight and ActualWidth returns 0 in the Loaded event just because the Scene element is not created at this time. You can handle the  Loaded event of the Scene instead e.g.

xamChart1.Scene.Loaded+=new RoutedEventHandler(Scene_Loaded);

private void Scene_Loaded(object sender, RoutedEventArgs e)
{ 
        double width = xamChart1.Scene.ActualWidth;
        double height = xamChart1.Scene.ActualHeight;
        MessageBox.Show(width.ToString() + " " + height.ToString());
} 

Let me know if you have any questions with this matter.

Vlad