I found a way to lower your in-game resolution below 800x600.
Now to do this you must set your pc’s resolution to 640x480 by right click>screen resolution>advanced settings>list all modes>640 by 480 then click apply after that run dirty bomb and go to the in-game settings and you get new choices of low resolutions.
Note : Increase the hud’s resolution or the game’s hud placement will be a mess
if you are done playing dirty bomb just right click then choose screen resolution and choose the resolution that you’ve been using before 640x480 for your pc
one more note : lowering resolution below 640x480 in game will cause the main menu not to show certain things such as the in-game settings button and to deal with that just click tab until you see it and it’s highlighted then press enter or click on it