I think 200 meters per hex might be the sweet spot. Long range weapon systems will exit the battlefield as units and re-appear as ground assets. We'll see...
I like the scale you are proposing for this battle.
This is a slugfest, but makes the Stalingrad battle compelling to me is the various terrain types, town, city, Mamayev Hill, factories, and wide gullies. I also like the variety of units; panzer, infantry, engineers, NKVD, all in a mix of guards, elite, regular, and militia types. The ability to capture the docks and stop Soviet reinforcements The player gets to decide where to put these units at critical points in the battle. In a smaller sized scale like this one, I would think the leaders would have more of an effect on units and the battle itself. I think this is the only city battle in history that I would like to play as a historical wargame.