Use the function
w32-send-sys-command. For example, you can
put the following in your .emacs file:
(add-hook 'emacs-startup-hook (lambda () (w32-send-sys-command ?\xF030)))
To avoid the slightly distracting visual effect of Emacs starting with its default frame size and then growing to fullscreen, you can add an ‘Emacs.Geometry’ entry to the Windows registry settings. See X Resources.
To compute the correct values for width and height, first maximize the
Emacs frame and then evaluate
(frame-width) with M-:.