When the game just opens in small window and you dont know what to do.
1. Go in your steam library and go on the settings of the game.
2. Open Local Files
3. Search for the "corpseparty" configuration settings type file
4. Open it (i used NotePad)
5. On ScreenWidth=___ ScreenHeight=___, write your screen resolution. For example: ScreenWidth=2560 ScreenHeight=1440
6. Save the file and open the game.
Now you should have the game on fullscreen and not on small window.