How can I set the game to custom resolution?

[Atreedee]

The game lags on all 16:9 resolutions and the 4:3 resolutions are so annoying and look so wide since they get stretched. So I want to set the game to a custom 16:9 resolution like 800x450 or something.
Edit: I tried editing the config file but the game crashed and when I ran it again, it was reset to 800x600.