Take a look at this repository dotMorten/WinUIEx.

It contains a method to set the window size and position

myWindow.SetWindowPositionAndSize(100, 100, 1024, 768);

I also found an example in the WinUI3 Samples I'm adding the relevant code here for easy reference

private void SetWindowSize(IntPtr hwnd, int width, int height)
{
    var dpi = PInvoke.User32.GetDpiForWindow(hwnd);
    float scalingFactor = (float)dpi / 96;
    width = (int)(width * scalingFactor);
    height = (int)(height * scalingFactor);

    PInvoke.User32.SetWindowPos(hwnd, PInvoke.User32.SpecialWindowHandles.HWND_TOP,
                                0, 0, width, height,
                                PInvoke.User32.SetWindowPosFlags.SWP_NOMOVE);
}

No need to do these interop calls on your own or use third-party packages for this.

Try this trifecta:

// Use 'this' rather than 'window' as variable if this is about the current window.
IntPtr hWnd = WinRT.Interop.WindowNative.GetWindowHandle(window);
var windowId = Microsoft.UI.Win32Interop.GetWindowIdFromWindow(hWnd);
var appWindow = Microsoft.UI.Windowing.AppWindow.GetFromWindowId(windowId);

Then you can finally set the size with:

appWindow.Resize(new Windows.Graphics.SizeInt32 { Width = 480, Height = 800 });

Note that an AppWindow object has several other functions as well, like MoveAndResize, Show, Hide, and features to modify the title bar.

I can't comment on answers yet but to add to Jonas' answer, you can put his answer in the class constructor (where this.InitializeComponent() is) of your main window code-behind or in the OnLaunched method of App.cs. I'm sure that seems obvious to a lot of people but to those coming from other languages/platforms it might not be. Example:

    public MainWindow()
    {
        this.InitializeComponent();
        
        IntPtr hWnd = WinRT.Interop.WindowNative.GetWindowHandle(this); // m_window in App.cs
        WindowId windowId = Win32Interop.GetWindowIdFromWindow(hWnd);
        AppWindow appWindow = AppWindow.GetFromWindowId(windowId);

        var size = new Windows.Graphics.SizeInt32();
        size.Width = 480;
        size.Height = 800;

        appWindow.Resize(size);
    // or like Jonas said:
    // appWindow.Resize(new Windows.Graphics.SizeInt32 { Width = 480, Height = 800 });
    }