Visually perhaps not, you may have 5,000 tiny little guys on a square but they'll still act as a single unit on the battlefield.
I'd like to see the battles exactly like those in Dom3 but under our turn-based control:
- before the battle, you can arrange your troops in formations as you want (divide your horsemen in 2 groups and place them on extreme left and right if that's what you want; make 3 groups of archers; put together your 3 companies of knights to make one batalion;...) and place them on the battlefield without knowing the adverse disposition
- during the battle, formations can't be changed and they place themselves on squares according to the numbers of troops and their relative size. Ex: in a square, you can put 50 peasants or 10 horsemen or 5 ogre or 1 dragon/big monster.
- you can give orders to single units or formations at the beginning of each battle turn
So, yes, if you put 5'000 tiny little guys together (to make one legion i.e.), they'll act almost as one (though each one fights individual and one after the other) because that's how it was. Watch Braveheart: there are guys with big flags besides the commander and, when they wave one, that means "Archers, fire!" or "Infantry, attack!". A formation can take many squares but they 'act as a single unit' in that all its components should obey to attack the same target (except when defending themselves if attacked. F***ing good battle mechanisms in Dom3!!) or go in the same direction or take the same stance.
if it works like that, that should alleviate your concern that 5'000 guys in one square can all attack one dragon on the next square: we would have those 5'000 guys divided into 100 squares (! Yes!! 50x100!!) and, as only 8 squares can border the dragon's square, only 400 of your 5'000 guys could attack at the same time but they must come, then surround it and then they prevent other units (say, your cavalry) to have a try against it while they are there.