Check this out:
Your symptoms don't sound exactly the same as mine, but could just be a different setup. My window was shrunk, so I had to use the suggested code to get the DisplayInformation and scale the code I used for the "actual" resolution size. I added the following 3 lines of code to achieve what I needed:
DisplayInformation d = DisplayInformation.GetForCurrentView();
this.actualHeight = (int)(this.actualHeight * d.RawPixelsPerViewPixel);
this.actualWidth = (int)(this.actualWidth * d.RawPixelsPerViewPixel);
Seems to have worked on the 4 inch, 5 inch 720p, 6 inch 1080p emulators, plus on local deployment to my 1440p screen.