Silverlight MVVM bind usercontrol to width/height

What do you mean by read only?

You can try using mode=OneWay when you define the binding.
This way, if the size of the usercontrol changes, it will not update the source

What I want is the opposite.  I want it only one way but from the usercontrol to the viewmodel.  If the usercontrol changes then the source will update.  The source will NEVER update the usercontrol as it is automatically done by the grid.  

So here is my scenario:
I have this usercontrol that I have created.  I have to get the ratio of the usercontrol against some physical size.  Since the usercontrol can be resized, I need to read the width and height values to calculate the correct ratio so I can scale everything inside the usercontrol accordingly.  All I want to do is just read usercontrol's width and height using binding.  

I figured it out!