TH information:
You need to edit the file "config.cfg", preferably with
textpad. If you don't have any expansion packs, you will find this file in the folder "Id1." If you do have expansion packs, you'll also find it in the folders "HIPNOTIC" and "ROGUE," and you'll need to edit all three of them to make TH work in all three games.
In config.cfg, find these lines:
vid_height "xxx"
vid_width "xxxx"
Edit them to set your resolution. Be careful not to mix up height and width.
In addition, find this line:
vid_conheight "xxx"
If it's not there, add it in. Set it to one third of your vertical resolution, rounding to the nearest integer. For instance, if you run 3840x1024, this would be 341. You're doing this so that the HUD doesn't stretch.
1024x768 4:3, vid_height "768", vid_width "1024", vid_conheight "256"
1920x698 "Ben-Hur" mode, vid_height "768", vid_width "1024", vid_conheight "256"