How do I change the aspect ratio. I tried using the command line like what the game said so, but it doesn't work (trying to change it to 1920x1080).

Won't work with the original executable, there were no widescreen monitors in 1997. You need to download unofficial patch 3.24 and unpack into the game folder, overwriting existing files.

http://kmq2.toastednet.org/downloads/q2-3.24-x86-full-ctf-1.5.zip