You can achieve that only by setting a few properties of the control:
and the needed size in the code behind:
.Width = System.Windows.SystemParameters.PrimaryScreenWidth;
.Height = System.Windows.SystemParameters.PrimaryScreenHeight;
However please not you would also need to modify the Template of the control a little bit to remove the dropping shadow for example. I have attached a sample project demonstrating the exact scenario, compared with the regular WPF Window.
Hope this helps.
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