Adjust the game's viewport-resolution to match your monitor's resolution.

Here we go - to adjust the game's  viewport-resolution to match your monitor's resolution, please follow these steps:

1. Open your "redorchestra.ini" file, you will find it normally on the following path: C:\Program Files (x86)\Steam\steamapps\common\Red Orchestra\System
2. Search for section [WinDrv.WindowsClient] . Edit the entry for FullscreenViewportX and FullscreenViewportY with the values that matches the "native" Resolution of your Monitor. Here an example for a simple 1920 x 1080 HD-Monitor =>
   FullscreenViewportX=1920 
   FullscreenViewportY=1080
3. That's all. Don't forget to save the file.. 

Restart Red orchestra and start a Single-Player (or Multiplayer) session. Watch your Monitor if the game-viewport is OK for you. If the gameplay becomes weird, the refresh-rate becomes too low, or you may have other issues in-game, just try a lower resolution settings or use the default settings available in "Configuration >> Display" in Red Orchestra.

! Don't worry, you cannot "destroy" something. If your Red Orchestra game will not start anymore after the changes you made, because you've done something wrong possibly, you just need to delete your "redorchestra.ini" before you start the game again. Of course you must recover all your game-settings after this action in consequence, because you reset the game to factory values with this action.
P.S. ..thats why I recommend to create a copy of your original "redorchestra.ini" file (name it "redorchestra.ini.backup" p.e..) before you change something on the original file.