You must open the game and change one of the graphics settings. After that, Width and Height should appear near the top of the INI file. You can then change the values. If you launch the game and it's on 16:9 and not 4:3 (in-game, the menu will stay at 4:3), it worked.

You can also change the FOV so it does not look weird, but you have to change the executable with an hex editor (as the free HxD). It is explained here: https://www.wsgf.org/dr/dungeon-siege. You might want to save a copy before doing the change.

The value to search wasn't the same for me as in the page (at least the end). But you should find it where there is 00 00 F0 41 00 00 70 42. replace 00 00 70 42 by 3a 5b 96 42 and save the file. But familiarize yourself with the main menu as it will flip the options :/. Though by hovering them, you will have the desc ription so you should be able to navigate in it without much problems.

I hope it's clear enough, english isn't my mother tongue.

EDIT: There's also a mod to change the resolution, improve the drawing distance, etc. https://www.moddb.com/games/dungeon-siege/downloads/seefar2020-increased-draw-distance-and-minor-improvements
It's the same guy who did the improved textures.