Is it just me or is this option just no longer available?
It's my default resolution, and 1080p looks fine and all, but the game runs MUCH smoother when it's at my native resolution.
You can achieve this resolution (in windowed mode at least) or any reasonable resolution by editing the d3prefs.txt file. I do this to get 1600x900 resolution windowed mode. Not an ideal way to get this resolution but it does indeed work.
You might have to do this, save the d3prefs.txt file every time before you open Diablo 3. But it seems if you run the game from the launcher and not from the game app, it saves the game resolution information most times. I don't know why this is though.