That's entirely up to how you want the scenario to play out. A lower average unit density/hex of front line makes for a more fluid game with breakthroughs more likely, a higher density makes it more likely the result will be a slow attritional grind. The number of moves each unit gets per turn has a similar effect. In most case you probably want to keep the latter between 8 and 20; but highly fluid scenarios like WW2 North Africa can benefit from much higher ones to better simulate the rapid sweeping advances that occurred after either side broke its rivals front line open and surged across the desert until they outran their supply lines.
Did you ever see history portrayed as an old man ... weighing all things in the balance of reason?
Is not [it] an eternal, imploring maiden, full of fire, with a burning heart and flaming soul, humanly warm and humanly beautiful?