I searched and found localization and config_mp
Alright, go into your main Call of Duty Modern Warfare 2, whereever it is (Probably in Program Files/Steam/steamapps/common if you bought it on Steam, or Program Files/Infinity Ward if you bought it physically)
Go into the folder called players.
Double click the text file called 'config' and if it asks what to open it with, open it with Notepad.
Press CTRL + F, and type this in:
seta r_mode
Now you'll see this:
seta r_mode "WIDTHxHEIGHT"
Replace WIDTH with your desired width value and HEIGHT with your desired height value. For example, I want to play in 1440x900 because I have a 19" widescreen monitor. I would put:
seta r_mode "1440x900"
Save, close, and launch the game.