查看GitHub上的this示例。
下面是一个WinUI 3窗口的完整示例，其中指定了最小和最大宽度和高度：

public sealed partial class MainWindow: Window
{
    private static WinProc newWndProc = null;
    private static IntPtr oldWndProc = IntPtr.Zero;
    private delegate IntPtr WinProc(IntPtr hWnd, WindowMessage Msg, IntPtr wParam, IntPtr lParam);

    [DllImport("User32.dll")]
    internal static extern int GetDpiForWindow(IntPtr hwnd);

    [DllImport("user32.dll", EntryPoint = "SetWindowLong")]
    private static extern int SetWindowLong32(IntPtr hWnd, WindowLongIndexFlags nIndex, WinProc newProc);

    [DllImport("user32.dll", EntryPoint = "SetWindowLongPtr")]
    private static extern IntPtr SetWindowLongPtr64(IntPtr hWnd, WindowLongIndexFlags nIndex, WinProc newProc);

    [DllImport("user32.dll")]
    private static extern IntPtr CallWindowProc(IntPtr lpPrevWndFunc, IntPtr hWnd, WindowMessage Msg, IntPtr wParam, IntPtr lParam);

    public static int MinWindowWidth { get; set; } = 900;
    public static int MaxWindowWidth { get; set; } = 1800;
    public static int MinWindowHeight { get; set; } = 600;
    public static int MaxWindowHeight { get; set; } = 1600;

    public MainWindow()
    {
        this.InitializeComponent();
        RegisterWindowMinMax(this);
    }

    private static void RegisterWindowMinMax(Window window)
    {
        var hwnd = GetWindowHandleForCurrentWindow(window);

        newWndProc = new WinProc(WndProc);
        oldWndProc = SetWindowLongPtr(hwnd, WindowLongIndexFlags.GWL_WNDPROC, newWndProc);
    }

    private static IntPtr GetWindowHandleForCurrentWindow(object target) =>
        WinRT.Interop.WindowNative.GetWindowHandle(target);

    private static IntPtr WndProc(IntPtr hWnd, WindowMessage Msg, IntPtr wParam, IntPtr lParam)
    {
        switch (Msg)
        {
            case WindowMessage.WM_GETMINMAXINFO:
                var dpi = GetDpiForWindow(hWnd);
                var scalingFactor = (float)dpi / 96;

                var minMaxInfo = Marshal.PtrToStructure<MINMAXINFO>(lParam);
                minMaxInfo.ptMinTrackSize.x = (int)(MinWindowWidth * scalingFactor);
                minMaxInfo.ptMaxTrackSize.x = (int)(MaxWindowWidth * scalingFactor);
                minMaxInfo.ptMinTrackSize.y = (int)(MinWindowHeight * scalingFactor);
                minMaxInfo.ptMaxTrackSize.y = (int)(MaxWindowHeight * scalingFactor);

                Marshal.StructureToPtr(minMaxInfo, lParam, true);
                break;

        }
        return CallWindowProc(oldWndProc, hWnd, Msg, wParam, lParam);
    }

    private static IntPtr SetWindowLongPtr(IntPtr hWnd, WindowLongIndexFlags nIndex, WinProc newProc)
    {
        if (IntPtr.Size == 8)
            return SetWindowLongPtr64(hWnd, nIndex, newProc);
        else
            return new IntPtr(SetWindowLong32(hWnd, nIndex, newProc));
    }

    private struct POINT
    {
        public int x;
        public int y;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct MINMAXINFO
    {
        public POINT ptReserved;
        public POINT ptMaxSize;
        public POINT ptMaxPosition;
        public POINT ptMinTrackSize;
        public POINT ptMaxTrackSize;
    }

    [Flags]
    private enum WindowLongIndexFlags : int
    {
        GWL_WNDPROC = -4,
    }

    private enum WindowMessage : int
    {
        WM_GETMINMAXINFO = 0x0024,
    }
}

如何在WinUI 3项目中使用C#指定WM_GETMINMAXINFO最小或最大窗口大小？
在Changed方法中有一种方法可以检测当前应用程序窗口的真实的大小，如果大小很小或很大，则可以调用Resize方法来设置可用大小。这里是如何获取appwindow的文档。