If you want to modify the resolution of OpenRA in fullscreen. All you have to do is locate the file settings.yaml (Under Windows XP it is located in C:\Documents and Settings\YOURUSERNAME\My Documents\OpenRA\) search for it on other operating systems.
Open settings.yaml in notepad or a text editor of your choice.
Make sure under Gaphics: it says something like this.
This only seems to work for me when resolutions are 4:3. If I try 16:9 resolutions like 854,480 or 960,540 then it bumps to a 4:3 resolution like 1024,768.
Also, the game crashes if there is a typo like "800x600" instead of using the comma: "800,600". Perhaps trouble in the YAML parser?