Are you sure that you’re running VCMI and not HoMM? VCMI has its own executables (VCMI_Client.exe and VCMI_Launcher.exe)
You can change resolution either:
a) Via Launcher - start VCMI_Launcher.exe, go to settings tab, select preferred resolution.
b) Via VCMI - start VCMI_Client.exe, start game, go to system options. There you should see your current resolution & button to change it.