To play in a wide window in high resolution, e.g. 1920x1080, you need to use the community patch (https://www.moddb.com/mods/warlords-battlecry-iii-widescreen-patch) and use gamescope. On first launch, after changing the resolution, the menu and hp bars in the game may glitch. To fix this, just run the first mission in the campaign and reset the game - now everything should work.