I've never needed to find out about setting the screen resolution, as I seem to always be installing Debian on laptops. But just now I installed on a desktop PC and the screen resolution was somewhat stuck on a low setting with no options in Preferences > Monitor Settings to set 1920x1080 pix.
So, whether its a propriety driver or its the kernel's driver that isn't detecting the monitor's capabilities, we might need to add a config file to tell the system what res to turn on the screen with.
What we need to do is edit the "Monitor" and "Screen" sections of xorg.conf - and I read that if that file is not around (in /etc/X11 - and its not there any more by default as X loads up device settings as it sees them during boot) it can be generated with,
or exit Openbox (ie stop any display manager running X) and run
But I only got an error and no file made.
The next option is to create a file in /etc/X11/xorg.conf.d/ with just the Sections that we want Xorg to make the settings for,
For my resolution requirement I put these lines in the file:
you can add modelines to Monitor section and modes to Screen section, as you need.
You get the modeline by this command
ie, `cvt <width> <height> <refresh-rate>`
If you are using a specific graphics card, then that might need adding too, with
Detailed manual is herewww.x.org/archive/...
A second monitor will need its own entries, and Monitor0, Screen0 and Device0 will change to Monitor1, Screen1 and Device1.