You can't.
You can however, specify custom resolutions via command-line.
Mine looks like:
"C:\Games\Battlefield 2\BF2.exe" +menu 1 +fullscreen 1 +restart 1 +szx 1280 +szy 768
Change the 1280 and 768 to 1280 and 1024.The +restart 1 is to get rid of those annoying intro videos.