I'm inclined to have some sort of logistical system which limits the number of UNITS (not people) in an army.
So level 1 would allow 4 units (a unit could be 50 guys still).
Level 2 would be 6 units
Level 3 would be 8 units
Level 4 would be 10 units
and then you'd have refined logistics which would add +1 every time.
This way, players who want to go the single big army route have to make some trade offs to do that.
Interesting idea, but it falls into the same throes as MoM. It looks like Elemental will have very linear warfare goals like MoM. For instance, "kill the sovereign" or "kill the capital" appear to both be what will end wars quickly. So what will we have? We will have one stack with all of our super units and a bunch of other stacks with weaker units clustering around that super unit stack like chicklets around a mother hen. They will move, in a big blobby mass, toward the enemy sovereign or capital at which point, we will play out a dozen or so tedious battles. Yay.
Emphasis on a big stack will shift toward an emphasis on a stack with super units. In essence, you trivialize armies that integrate weak and strong units. So I propose an amendment to the idea. Instead of a hard cap based on logistics, institute penalties if you go over the cap. For instance, let's say the cap is 500 units at some point in the game. If you stay within that cap there is a small green bar next to your army that represents that all is well. Now, if you go over that cap by say, 100 units, the bar is light green. This represents that your soldiers are taking a small movement and moral penalty.
Now, if you more than double your logistics cap again, you end up with a yellow bar. More penalties. More than triple the cap? Orange. You get the picture. The more you exceed the cap, the more penalties that army suffers. So sure, the player will have to choose between whether they want one super dooper stack that has a nasty red penalty or lither armies without any penalties with which they can strike multiple strategic targets. Ultimately, though, there will be a point where the player will say, "you know what, it's just not worth stacking penalties in this stack anymore." And this is what you could call the "soft cap" on stacks.
This way, you can still rely on numbers rather than only elites. And what's more, it opens up some strategic venues. You might decide to rely on multiple green stacks to strike strategic targets. You might rely on one super stack of elites to keep your army "green barred." Or you might forget logistics completely and stack zounds of peasants into an army who have to march through their own filth.