As I wrote on page 3 of this thread, May 17, 2016, 10:35 AM:
The preferences.txt file should be located here:
..\Rome Total War Gold\preferences\preferences.txt
For BI it should be here:
..\Rome Total War Gold\bi\preferences\preferences.txt
For Alex. it should be here:
..\Rome Total War Alexander\alexander\preferences\preferences.txt
The first folders may vary based on your disc version of the game and how you installed the game (the "Rome Total War Gold" and "Rome Total War Alexander" ones).
However, the .txt files will only be created if you succesfully started the game at least once.
If you have a new install, you obviously haven't been able to start the game succesfully yet, and therefor you should create the preferences.txt file yourself.
Just create a .txt file called "preferences.txt" and put this in it to start out with and save the file:
NOTE: The part I put in bold text below, is what you should edit to match your own specs, according to my guide on the first page of this thread.