Change size and position when a window is activated. For MAUI on Windows platform.

maui-change-window-size-position.cs

public partial class App : Application
{
    public App()
    {
        InitializeComponent();

        MainPage = new AppShell();    
    }

    protected override Window CreateWindow(IActivationState activationState)
    {
        Window window = base.CreateWindow(activationState);
        window.Activated += Window_Activated;
        return window;
    }

    private async void Window_Activated(object sender, EventArgs e)
    {
    #if WINDOWS
        const int DefaultWidth = 1024;
        const int DefaultHeight = 800;

        var window = sender as Window;

        // 變更視窗大小
        window.Width = DefaultWidth;
        window.Height = DefaultHeight;

        // 給它一點時間來把「變更視窗大小」的操作確實完成。
        await window.Dispatcher.DispatchAsync(() => { });

        // 取得主要顯示器的資訊
        var disp = DeviceDisplay.Current.MainDisplayInfo;

        // 將視窗置於螢幕中央
        window.X = (disp.Width / disp.Density - window.Width) / 2;
        window.Y = (disp.Height / disp.Density - window.Height) / 2;
    #endif    
    }
}