Prey (2006): Tips (How to Set Game Resolution to Your Monitor)
How to set the resolution of the game to your monitor?
If permission 1600×900 is not installed, then it can be set manually by editing the file Steam \ steamapps \ common \ Prey \ base \ preyconfig.cfg:
seta r_customHeight “900”
seta r_customWidth “1600”
seta r_mode “-1”