This guide should help you save your video resolution setting and should stop it re-setting itself every time you load the game.
Guide to Save Video Settings
C:/Program Files(x86)/Steam/Userdata/You Personal ID number folder (folder with numbers/308420(find folder with this number)/Remote..
Once in this last folder you should find: global_config.xml
Right click this file and open with notepad and set your resolution to what you want, save and exit.
You should be good after this.