Baseline (Widescreen) Solution & Issues
Select your resolution from the Settings, Graphics menu then quit & restart the game.
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
Eyefinity / Surround Specific Solution & Issues
Select your resolution from the Settings, Graphics menu then quit & restart the game. On my system (Nvidia cards) I also had to turn HDR off or the game would be very dark in multi-mon.
When zooming out, most of the map is not rendered beyond the central 16:9 portion of the display.
To fix this, use the Ultimate Camera mod and lower the Distance factor. See Additional screenshots below.
4k UHD Specific Solution & Issues