The standard game is around 900mb installed, and Dark Avatar (soon to be released) will add an additional 250mb or so to that. If you install the game via Stardock Central, however, you can opt to exclude the videos and sounds to cut the space requirement of the base game by half or more.
Note however that depending on your system's configuration, you may need to increase your windows virtual memory size. At least 1gb for virtual memory is a good idea, preferably 2gb or more, especially if you have less than 1gb of RAM. If your virtual memory and system memory combined are too small, you may experience crashes when playing on larger maps.
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