Change resolution on Desktop to some lower and differ aspect.
If you have Intel+Nvidia it can be set from Intel (Notebook's and else mobile devices).
Should be appear Keep Aspect ratio option.
For game itself... It support wide resolution as i remeber, but with some incorrect scale\FOV. For some aspect (16:9, it probably support 16:10 but this didnt help if you have 16:9 display) No updates for long time even for GOG builds (just gog files).
Check for more info
http://www.pcgamingwiki.com/wiki/Heroes_of_Might_and_Magic_V (also link to WSGF)
Also, Tribes of the East - is compeltely separate game, separate installment. Just same engine, but differ game. Or a little bit differ engine. And it have more proper widescreen support for more aspect ratio's.
Btw, TotE page
[url=http://www.pcgamingwiki.com/wiki/Heroes_of_Might_and_Magic_V:_Tribes_of_the_East]http://www.pcgamingwiki.com/wiki/Heroes_of_Might_and_Magic_V:_Tribes_of_the_East[/url]