SilverLaw has created a Silverlight 3 behavior that provides resizing for a regular Silverlight 3 ChildWindow using mouse wheel scroll including Blend 3 design time support.
Short description:
1. Import System.Windows.Interactivity.dll.
2. Add a reference to the .dll in the download.
3. Drag the behavior from the asset panel onto a ChildWindow.
4. Set a fixed width and height to the ChildWindow. Set width and height of the Layoutroot to Auto.