Vista:
C:\Users\Username\AppData\Local\Ironclad Games\Sins of a Solar Empire\Setting\user.setting
XP:
C:\Documents and Settings\Username\Local Settings\Application Data\Ironclad Games\Sins of a Solar Empire\Setting\user.setting
Please note that some of these directories are hidden by default. To unhide them, go to Control Panel -> Folder Options -> View and set yourself to view hidden files and folders.
Look for:
VideoFullScreenWidth
VideoFullScreenHeight
VideoWindowedWidth
VideoWindowedHeight
They'll be near the top. Line 15 or so.
