Check out example codes for "set uwp page size when opened c#". It will help you in understanding the concepts better.

Code Example 1

public MainPage()
{
    this.InitializeComponent();

    ApplicationView.PreferredLaunchViewSize = new Size(480, 800);
    ApplicationView.PreferredLaunchWindowingMode = ApplicationViewWindowingMode.PreferredLaunchViewSize;
}

Code Example 2

//You don't really have control over the window size,
// and even if you will try to re-size it it may fail.

// Place the next code on your page's "Loaded" Event
float DPI = Windows.Graphics.Display.DisplayInformation.GetForCurrentView().LogicalDpi;

Windows.UI.ViewManagement.ApplicationView.PreferredLaunchWindowingMode = Windows.UI.ViewManagement.ApplicationViewWindowingMode.PreferredLaunchViewSize;

var desiredSize = new Windows.Foundation.Size(((float)800 * 96.0f / DPI), ((float)600 * 96.0f / DPI));

Windows.UI.ViewManagement.ApplicationView.PreferredLaunchViewSize = desiredSize;

Window.Current.Activate();

bool result = Windows.UI.ViewManagement.ApplicationView.GetForCurrentView().TryResizeView(desiredSize);

//In the "desiredSize" calculation, 800 is the width and 600 is the height.
// This calculation is needed, because the size is in DPI,
// so you have to convert it from pixels to DPI.

// Also keep in mind that size cannot be smaller than "320x200".

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