title: Get the size (after it has been "stretched") of an item in a ViewBox
body: <p>Consider the following:</p>
  
  <p>
    
      
    
   </p>
  
  <p>Let's say the Window is 1024x768 and the ViewBox fills the entire window,
  this means the TextBox is really large on the screen.</p>
  
  <p>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.</p>
  
  <p>Any suggestions?</p>
  
  <p><strong>Update:</strong> I could probably get the <code>ActualWidth</code> of the ViewBox and divide it by the <code>ActualWidth</code> 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.</p>