There are no 'cfg' or 'ini' files to edit when it comes to settings. So, you're limited with the options available in the games settings for graphics resolution. However, the resolution you play at 'should' not be making much difference in performance... if that's what you are going for. For this game, about the only way you'd see much difference in performance is if you are going larger than say 1920 x 1080, like on a 4k res monitor for example.

Some of us doing some performance testing on the CMS 2018 game actually found the game ran 'slightly' better at the 1920 x 1080 res vs a lower one like 1600 x 900 for example. Why? I'd conclude the devs must have been optimizing the game for that resolution but, that's just an educated guess. Ofc, this testing was done on hardware configuration(s) that exceed the games minimum requirements. So, a PC that doesn't meet minimum requirements may see more noticeable changes in performance trying different settings. That said, the newer 2021 version does seem to perform better than the previous 2018 version.

Maybe if you tell us what PC specs you have and what you hope to achieve, there might be some better info for you.