Software should adapt to the size/aspect ratio that the user gives to the application
A couple of fun games built on Silverlight:
1) ZeroGravity: http://ltbennett.com
2) Destroy All: http://andybeaulieu.com/silverlight/DestroyAll/ (I like how you can play the game using images from Virtual Earth!)
Notice that they take very different approaches to sizing. ZeroGravity takes all available space based on the size that the user gives the browser window. DestroyAll has a fixed size…which looks really lame on my 1600×1200 monitor.