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It looks like if you're using Windows, it's as easy as adding this line into your .vimrc:
au GUIEnter * simalt ~x
(type :help maximize or :help win16-maximized for details)
Apparently there's no standard way to do it on other platforms from your vimrc. The simplest trick without knowing which platform is to add
set lines=999 columns=999
to your .gvimrc. There are a few other tricks that can help at the Vim Wiki's Maximize or set initial window size page.