Hi applebutter,
Sorry to hear you are having this problem. Have you tried setting the resolution manually in Options.ini located at:
~/Library/Preferences/Command and Conquer - Red Alert 3 Preferences/p_drive/User/Applications Data/Red Alert 3/Profiles/<Your Profile>/Options.ini ? Could be that you didn't save the file after editing it?
If that doesn't work, I would suggest opening the 'config' file located at ~/Library/Preferences/Command and Conquer - Red Alert 3 Preferences/ folder and under the [sdldrv] section, you will see "Desktop" = "N". Change that to "Desktop" = "1920x1080" or any other resolution you want the game to start up in. Note that this will cause the game to launch in windowed mode. You can switch to fullscreen mode via Cmd+Enter.
Hope this helps. If the issue persists, please contact us at "support AT transgaming DOT com" and I'll be more than happy to investigate this further.