Baseline (Widescreen) Solution & Issues
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
Eyefinity / Surround Specific Solution & Issues
The game will start at your native resolution no problem.
However if you wish to play on a single screen, the list of available resolutions in-game will probably be truncated, so you will need to edit a file instead.
(Win7 x64) C:\Users\<username>\AppData\Roaming\Might & Magic Heroes VI - Game Official Demo\Save\<profile>\ProfileData
Remember this is for the DEMO, the full game's save folder may be simply Documents\Might & Magic Heroes VI.
The file ProfileData has no extension but can be opened in any text editor. Edit the following line:
<gfx_Resolution>5760x1080</gfx_Resolution>
4k UHD Specific Solution & Issues