Use this method to get the screen size:
public static Size GetScreenResolutionInfo()
var applicationView = ApplicationView.GetForCurrentView();
var displayInformation = DisplayInformation.GetForCurrentView();
var bounds = applicationView.VisibleBounds;
var scale = displayInformation.RawPixelsPerViewPixel;
var size = new Size(bounds.Width * scale, bounds.Height * scale);
You should call this method from App.xaml.cs, after Window.Current.Activate(); in OnLaunched method.
Here's the sample code and you can download the full project.