So, you added
/fullscreen 1280 720
in the Supreme Commander options of GPGnet and then hosted or joined a game?
This doesn't work. Supreme Commander seems to ignore the parameters that are added.
Open the supreme commander demo directory by right clicking on the demo icon on the desktop and clicking properties then click find target under the shortcut tab and in the folder you'll see a shortcut file named "data files" right click the icon and select properties. In the menu under the shortcut tab click the button find target, this will open a new window with the folder named SupremeCommanderDemo. Inside is a file called "Game" open the file with notepad and find "primary_adapter = 1024,768,60'," and replace the first two with the resolution and the last for the Hertz if your screen can go higher than 60. When you try to change video options while in the game it will try to put the resolution back to the default setting is all you have to do to keep the changed setting is tell it you don't want to keep the new resolution setting.
it took me about an hour of searching the net to find nothing about this except for here, hope it helps. And the reason I didn't tell you exactly where the file was is because the folder Application Data is hidden and this was much faster any ways. Have fun
This works. Thanks!
P.S. I am playing with the retail version with the latest patch, single player.