"seta r_customwidth #" where # is the width (X) of the resolution
you wish to use.
"seta r_customheight #" where # is the height (Y) of the resolution
you wish to use.
"seta r_mode -1"
After doing this, you'll need to restart the video subsystem for the
change to take effect. Do this with the following console command:
"vid_restart"
seta or set?