Beginning with Emacs 24.4 either run Emacs with the ‘--maximized’ command-line option or put the following form in your .emacs file:
(add-hook 'emacs-startup-hook 'toggle-frame-maximized)
With older versions 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-height) and
(frame-width) with M-:.