Go to <gameDir>\system. Open deusex.ini. Find these lines:
Change them to set your resolution.
Now open user.ini. Find these lines:
Change the values to set your FOV to your preference. I recommend 85 for 16:10 resolutions and 90 for 16:9 resolutions.
This game is Hor+, dependant on your FOV setting.
If your resolution constantly resets in user.ini:
First run the game and select a 4x4 digit res from the video options menu, i.e 1600x1200 or 1280x1024. Then you can edit user.ini and your new resolution will stick.
Deux Ex Widescreen Utility:
Handy tool to instantly adjust the resolution, gameplay FOV and UI scale - but only supports 16:10 and 16:9.
To the best of my knowledge, the FOV setting affects every part of the gameplay that uses the 3D view. 2D gameplay elements, mainly the extensive screens used for things like managing inventory, augments, and logs, are scaled correctly.
However, in the cut-scenes, at some of the higher resolutions, parts of the subtitles can get cut off. Dialog is all spoken, so this shouldn't hurt playability.
The cut-scenes are rendered ingame. Unexpectedly, they are anamorphic, independent of your FOV setting.