I don't really even understand the concept of troop size.  What exactly are we researching?  

 "You know guys...I was thinking...our troops are pretty good, they work pretty solidly in groups of three...but I was thinking that they would be even more powerful in groups of five."

"What?!?  Five?  Is that even possible?  How could five people work together to kill things?  No, you dream to large, all science points to 3 being the top number soldiers can group together for the same purpose"

"No, I think I am really on to something.  I first realized that a unit of five is better than a unit of three when I realsed five is a bigger number than three"

"That's just a theory!"

"Hear me out!  I've done the math over and over and every time, I get five being larger number and that five soldiers would do more damage than three soldiers"

"That's incredible if true!  It will take years of research to even begin trials into this theory."

"I know, but it will change the world!"

"It will!  But why five, and not four or six?"

"Fuck off"

After hundreds of years of research, they get to 7, I've heard rumors of 9, but never actually seen it myself...

Lord Xia, that was great! 

Not to detract from Lord Xia's brilliant Monty-Pythonesque satire, but is there something that could be researched that would make sense here?  Logistics (ala GC2)?  Small unit coordination?  Command structure?  Magic-based unit communications?

Once I recovered from my Xia-induced laughing fit, I agree with OP.  And I think it has been requested before.

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

Good, then its probably gonna be implemented eventually .

That's exactly how I've always figured it working.  The troop size techs include leadership training, communications streamlining, and enhanced tactics.  Disregard the fact that you're coordinating three/five/seven guys, and it makes sense (to me anyway).  The tech enhances the command structures and operations to effectively coordinate larger groups of fighters.

was going to suggest the same thing... OP beat me to it.

I would like the option of upgrading my troop size, IF the total amount of experience gained by the unit was divided appropriately. For example, if I upgrade a 3-man unit to a 7-man unit, it should have its current experience divided so: Current XP * (3/7). This is to symbolize bringing fresh recruits into a battlehardened unit, greatly reducing its effective combat power.

Yes, but there is also a per soldier cost in the ElementalDefs.xml So a unit costs according to the number of soldiers. 

I'm not sure if it would be worth it, but this could be exploited to crank out troops faster while only 'half-hurrying' production:

Build the smallest size troop, no matter how big you want the unit to eventually be.

Use gold to upgrade the troop size.

This is definitely exploitable if the cost to upgrade a unit is significantly less than the cost of rush-building it at the size you want. In fact, this would be exploitable even more in the long run in terms of unit maintenance: just keep the smallest troop size garrisoned until you need them so you don't have to pay so much for them, them when you want to use them, upgrade them. Of course, this approach can already be used to some extent with the available upgrade abilities.

One way to make this a tactically risky strategy might be to require upgrades to also immobilize a unit for a few turns, depending on the degree of upgrade, similarly to when heros die and end up immobilized in the capital.

I suggested this in https://forums.elementalgame.com/418536:

"It could be, for experience of the new unit, and the amount of building turns (as percentage of new unit):

New Experience = (Original Size/New Size) x Old Experience

Training Time = (Original Size/New Size) x Normal Training Time for new unit of bigger size

which would take care of the new units needing training, and also starting with no experience.  I don't think they should be also able to upgrade equipment at the same time though."

So the unit would be immobilised in a city for that Training Time based on the city they're in, and if attacked before ready probably the old unit would defend (and if the old unit survives and gets extra experience it would be taken into account in the "New Experience" calculation above).  Maybe also group number upgrades couldn't be rushed by Call to Arms.