How to save console options in Portal 2?

It's not recommended to modify config.cfg, since it can easily be overwritten by the game. The proper file to write your scripts and commands is autoexec.cfg which should be located in the same folder ([steam location]\steamapps\common\portal 2\portal2\cfg). If it's not there - create it. This file is executed automaticly when you load a map.

cl_fov 75;

should solve your problem.

In your Steam folder for the game, there's a folder inside the "portal2" folder called "cfg" (I think the default C:\Program Files\Steam[usernamehere]\Portal 2\portal2\cfg) inside that folder there's a file called "config.cfg".

In that file there's a line called fov_desired "90.000000"

Changing that to 75 might work.

If not, then adding "cl_fov_75" as another line at the end should.