Change Aspect Ratio (21:9 Ultrawide, 4:3 Nostalgia)

Quick and easy method to force the game to run in whatever aspect ratio you want.

Set launch commands

In your Steam library, right click on the game and select Properties. In the new window that pops up, look at the bottom for the text field for launch commands. There, enter:

 -screen-width 3440 -screen-height 1440 -popupwindow 

Change the numerical value for width and height to match your desired resolution.

That's it! Just launch the game normally and enjoy ultrawide goodness or classic 4:3 Nostalgia!

[bootcutjorts]

@Pacomatic The windowed setting doesn’t work for me on the Deck, it just remains at 16:9 with bars on top and bottom. And whenever I choose the 16:10 setting, it doesn’t fit to the screen, so there’s black bars all around it. Not sure if it’s a bug or if I’m doing something wrong

The above solution for launch options did fix it for me, but still is an annoying work around if I’m switching back and forth between docked and handheld

[Pacomatic]

This is no longer needed, as you can set aspect ratio to "window".
Great for 48:9, and it seems to stretch as much as you want; I set the gamer to windowed, and squashed it to 1920x43.
No issues, other than the nauseating FOV and the extremely low resolution.