Set window size of UWP application

I am writing a cross-platform (iOS, Android, UWP) application using PCL and Xamarin.Forms.

I use the UWP app for general testing since it is easier (don't need an emulator).

Is there a way to change the default window size of the UWP? I'd like it more "phone" shaped. It will help with estimating the layout (realizing I still may need to tweak on each platform).