If your screen has patterned grey sides
then it has been shrunk accidentally by a keypress or cfg change.If you are running in full screen mode, and the borders are plain black
bind - "sizedown"
bind = "sizeup"
is default (the -/_ and +/= keys above the alpha keys) not the KP_ keys (numeric keypad to the size)
you can just set
in the console instead, as that is the maximum, and that is what sizup/down changes.
then that will have no effect, black borders are caused from using resolutions that do not scale to the size of your monitor. You will either need to use a different game resolution that matches your screen (use the in game menu via pressing escape), or get your monitor or gfx card drivers to enable scaling.
If you can't find a scaling option, then you're only option is to use your screens native resolution, it will be the same resolution that your operating system desktop is running at or you would have borders for that also. If the resolution you need is not in the r_mode list then just try using the console to set
and that should use your desktop resolution.