Sorry, but I wasn't able to reproduce the behaviour you are describing. However, I can suggest you even better approach - instead of writing GetToolWindow() method, you could use the build-in one, which is faster and more clearer.
sender, Telerik.Windows.RadRoutedEventArgs e)
var pane = e.OriginalSource
var toolWindow = pane.GetParentToolWindow();
toolWindow.Height = 100;
toolWindow.Width = 100;
Please try with it and let us know if that resolved the issue. If you still experience it, could you please try to reproduce it in a sample project and send it back to us. In that way we will be able to track down the source of the problem and provide you with a solution in a timely manner.
the Telerik team
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