I'm playing on a 1080p 16:9 monitor. I felt that the games default 4:3 resolutions look a bit too small so I managed to add a command line to a custom executables via gog galaxy so I can play at 16:10 resolution (1680x1050) and the game looks pretty great now, full screen except some small black borders at the edge. So it's not as stretched out as a full 16:9 resolution would be, and in my opinion gives an optimal look for this game on a monitor of this type.

My command line in the custom executables looks like this,

-CustomResolution:enabled -r1680x1050x32 -CPUCount:1

Managed to get 1920x1440 and using currently with 4 cores, yet not very long, so dunno about the cores and stability. Time will tell.

-CPUCount:4 -f -CustomResolution:enabled -r1920x1440x32

It poops out when I try 3440x1440. I will try 1720x720 to achieve double-pixel as I do with Heroes III HD+

EDIT: with 1720x720 it ends up 4:3 anyways. So no ultrawide for now.