You can put this code in the constructor to do what you want:
Form form = (Form)Control.FromHandle(Window.Handle);
form.WindowState = FormWindowState.Maximized;
This requires that you add a reference to System.Windows.Forms
to your project.
This does have the downside that it actually sets up the graphics device twice on startup. Once in the normal way, and then once again because the window was resized. (Although this all happens before the form is first displayed.)
It has the advantage of being extremely simple to implement.
I'm going to poke around for awhile and see if the initialisation order can be changed...
There really is no simple way to get around this problem, it would seem.