1. Create a brand new project and add a ShapedForm.

2. Set the Size to 800,550.

3. Set the FormBorderStyle to FixedToolWindow.

You will notice that at run-time the size is changed to 816, 589.

Workaround:

        public ShapedForm1()
        {
            InitializeComponent();

            this.MaximumSize = new Size(800, 550);
        }