You can get the client size via the the window visual tree root’s ActualWidth/Height, e.g,
FrameworkElement f = VisualTreeHelper.GetChild(window, 0) as FrameworkElement;
f.ActualWidth/Height
In Window’s default style, the visual tree root is a Border.
If your content is FrameworkElement and its layout behavior is sized to container, you can get the ActualWidth/Height directly of window.Content.