I copied this part out of the Readme.txt form the Civilization folder (not the play the world folder):
*KeepRes=1 - When set to 1, your desktop resolution will be what the game uses. 1024x768 screens will be centered, but you will be able to see much more of the map on one screen.
*NoSound=1 - Disables all sound in-game.
*Video Mode=1792 / Video Mode=1600 / Video Mode=1280 / Video Mode=1152 - Force screen resolution to one of these settings.
But I must admit that the Video Mode=XXXX doesn't work on my computer with my PTW- version of CivIII and Windows XP. I don't know why. The KeepRes=1 does work, so it isn't a problem for me. I set the resolution at my desktop at 1152*864 and Civ III looks nice at this resolution.