Olá amigo,
Tente dessa maneira:
string larguraDaTela = Application.Current.Host.Content.ActualWidth.ToString();
string alturadaTela= Application.Current.Host.Content.ActualHeight.ToString();
Dê uma olhada nesse tutorial
http://programmerpayback.com/2013/08/31/detecting-screen-resolution-in-windows-8-and-windows-phone-apps/