1. Check your screen name

Open a terminal and type this command:

You’ll get a list of available screen sizes, along with the name of your screen. In my case, that’s default.

2. Run xrandr as an OpenBox startup command

Open the ~/.config/openbox/autostart file, and add this line:
xrandr --output {screen name} --mode {window size} &

xrandr --output default --mode 1280x800 &

That’s it! OpenBox will now use your desired screen resolution at startup.