Hi,
In WPF there is concept of relative width ('auto' and *) apart form fixed pixel width.
To make sure that your app reacts to the available screen space and adjust the inner control accordingly then first thing you must do is
do not use fixed widths.
Before designing your app (in blend or in VS), you must be aware of the Measure and Arrange cycles of the WPF application. This will help you understand how WPF architecture actually layouts your controls (this is a very different concept then that of Paint in Winforms).
The link below could be a good place to start with:
http://msdn.microsoft.com/en-us/library/aa970268.aspx#Layout[
^]
Thanks
-a