Probably something on the level of advanced command structures ... but a bit of realism on that would not lend itself well for this type of game imho.
To make it a bit realistic, I think you'd have to (instead of troop size) select a type of formation ... and each formation does well with different numbers of troops.
First you start out with the wild mob ... anywhere from 10 soldiers to 100 soldiers is acceptable. An even keel throughout the whole thing. Just loosely organized, only those in the front get to attack.
Then you get better ones...
Phalanx-> 20-60 soldiers ... really weak to flanking/rear attacks, but incredibly strong straight on. Impenetrable by Cavalry. Spears only (Spears and Shields preferably). EVERYONE gets to attack, but you have ZERO defense if someone gets behind you ... and you can't change direction easily.
Cohort -> 20-80 soldiers. Looser than the Phalanx, yet still tightly organized. Better Air Dodge from shields. Is faster than the Phalanx, and much more effective than the mob.
The Cohort loses its effectiveness if attacking a Phalanx head on, but they have the power of maneuverability, and are much more well defended against flanking. Not all soldiers can attack (half in front or back, a quarter on the sides)
Basically the Cohort is the ideal defense against both archers and being surrounded by light cavalry. Heavy cavalry will still squash to Cohort from head on however ... as they do not have any inherent bonuses vs Horses.
Skirmishers --> 10 - 40 soldiers the Skirmisher is the ultimate maneuverability formation. A common choice for melee, light cavalry, and ranged units.
Provides extra Air Dodge vs Ranged attacks even WITHOUT the Shields
Provides +1 tactical movement
All units may attack, although the number of soldiers is limited because spread too thin and they won't know what orders to follow.
All soldiers may attack in any direction ... yet soldiers also get a penalty to defense (which doesn't matter if no armor to begin with! )
Wedge Formation --> 20 - 40 soldiers, Cavalry only. All soldiers may attack forwards, yet only half in any other direction. Low soldier limit because horses are expensive and stronger (I think its an artificial reason at best, but meh)
A slight defense penalty when attacked from the rear, as it indicates the unit being "surrounded" so to speak, or else the cavalry is trying to break from combat and get away.
Cantalabrian Circle --> 20 - 40 soldiers. Also Cavalry only. And Ranged Cavalry only at that! Starts out with an initiative bonus that later turns into an Initiative negative as the battle drags on. At the start, Air Dodge is more than doubled (very high) ... and then later starts to bleed off at a faster pace then the initiative does. However it approaches zero, and does not go into the negative. Basically used for Horse Archers vs Archers, or Horse Archers vs other Horse Archers. perhaps a slight decrease in accuracy, as they are riding in circles? And possibly -1 tactical movement UNLESS mounts still give only +1 move.
The Horde --> 60 - 200 soldiers. The late game s answer to the Wild Mob. Unlike the Mob, Cavalry can also use this formation ... especially horse archers
+1 strategic movement. The Hordes tend to fight on their stomachs, yet instead of a supply line they loot and pillage for their food, thus increasing their travel time (plus who likes slow Mongolians? The strategic advance must be quick, but tactical should not be overwhelming)
50% less maintenance costs .... The Horde is not a whole lot better than the Mob, although melee Hordes get a bonus to frontal attacks/charges. The main advantage is slightly strategic boost, and cheaper units. If you make stacks entirely of hordes, you can cover ground quickly and have a cheap army ... the late game's answer to reliance on super units
You can adjust the slider to say how many soldiers you want the max to be (inside the allowable range).
Really, you just stuff as many people into the unit as you feel you can maintain maintenance for, and that you feel you can wait out the production times for as well.
Then just crank up unit production speed, make Heroes stronger to compensate for the massive army sizes, and VOILA! Awesomeness.
Still, I know this won't make it into the game, so its only a pipe dream