It involves editing a .cfg file in the game's folder in order to modify the screen resolution to a custom value.
QUoted from somewhere:
Open the DoomConfig.cfg file in Notepad
Then scroll down until you get to a section with "seta r_*" commands. Find and change the values as I have listed them below:
seta r_customHeight "1050"
seta r_customWidth "1680"
seta r_mode "-1"
These should work.