When I plug an external monitor into my MacBook (Air, Pro... it doesn't matter), I can change the monitor's settings (resolution for example) and Mac OS X saves the configuration for the next times I plug it. Is there anyway I can reset those settings? Any file to delete?
Yes, you can reset your display settings by deleting a certain preference file located here:
Once in that directory, look for a file that begins with
HOWEVER, there is a caveat. Deleting this file can have serious consequences. If I were you, I wouldn't mess with it.
If you're determined to try, then edit the file, only removing the dictionary in the plist that contains the screen settings for the external screen. If it sounds too technical, then I wouldn't try it. Troubleshooting a non-functional system because you deleted a system file just isn't fun.
as well as
Put both in trash.
This most likely will not change your graphics settings; however, it was my experience that doing this whole process in "scaled" display from the beginning, allowed me to go back in to settings after restart, and choose "Best for display" and resolve my issue.