Nicely done. Just mirroring this from the link just in case!
What you have to do (in this, JO and any other Q3A-based game) is go into your base folder and open "jaconfig.cfg" (and "jampconfig.cfg" for multiplayer) in Wordpad (not Notepad, as the file contains line breaks that Notepad can't interpret). Scroll down and edit the following lines to the red values:
seta r_mode "-1" * This is crucial. If you don't change it, none of the next settings will take effect
seta r_customwidth "1440" * Or whatever the width of the desired resolution is
seta r_customheight "900" * Or whatever the height of the desired resolution is
Also, if it's not there, you should modify the following to a value between 90 and 105 (whichever looks better to you).. to keep everything in proper perspective:
seta cg_fov "100"
** Note: You will have do do this in every mod's folder aswell **