Whoa! Here is a feature that slipped through cracks for me but is something I have been asking for a long time… I just didnt know they got it into Blend 2.
It used to be in Blend that you had to explicitely set a width and height to see your content render (specifically around root elements like Windows and Pages). This was problematic because inevitably I would check in my xaml with these widths and heights set which would usually break resize scenarios.
Now with the design-time size you can set a designer only property that has no effect at run time. Just look for the bar and triangle glyphs as shown below and resize your content.
This is what it adds in xaml: