All settings can be changed by starting the game via a command-line parameter like
If you create the folder
Support relative to the OpenRA main directory, everything including settings gets stored there to aid portable installations.
Player: Name: Newbie
Graphics: Mode: Fullscreen FullscreenSize: <CUSTOM WIDTH>,<CUSTOM HEIGHT>
Graphics: Mode: Windowed WindowedSize: 1440,900
Debug: LongTickThreshold: 0.001
Debug: IgnoreVersionMismatch: True