UIElement.OnRenderSizeChanged

When overridden in a derived class, participates in rendering operations that are directed by the layout system.This method is invoked after layout update, and before rendering, if the element's RenderSize has changed as a result of layout update.

This method has no default implementation.A typical WPF framework-level element uses the override implementation from FrameworkElement.The FrameworkElement implementation invalidates the Width and Height properties and handles the basics of remaking the layout.Overriding OnRenderSizeChanged  at the UIElement  level implies that your element implementation is deliberately not using the WPF framework-level implementation, and therefore your element must handle layout logic more directly, typically by writing a replacement layout system.