Get the size (after it has been "streched") of an item in a ViewBox

Question

Consider the following:

Let's say the Window is 1024x768 and the ViewBox fills the entire window, this means the TextBox is really large on the screen.

I want to get the size of the TextBox as it is currently on the screen. If I get DesiredSize or ActualSize or even RenderedSize I always get 100.

Any suggestions?

Update: I could probably get the ActualWidth of the ViewBox and devide it by the ActualWidth of it's child which would give me the current scale factor and expose that as a property somehow but I'm not sure that's the best way to do it.