Check out example codes for "defualtsize UWP 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;
}

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