Just a quick answer to your post and I'm in no way trying to counter what you want but,

The demo is availiable for free and download (hopefully you have a decent connection).

There is a map, made by Gandalf (Yay) that gives you a bunch of provinces and 2 castles enough that you can field different types of army (small, medium, large, based on what you are looking to see if fits you) way before the end of the demo and then you can send your armies to battle an opponent, or drive towards a castle.

You *should* run into an army of comparable size some time along the way that will give you the feel of the combat.

I personally don't like 3 Elves and a Swordsman conquored a town mentality, but I don't feel that Dom2 fits that bill. I've had massive battles and heroic encounters where a few units have bested a horde of enemies. It may not be on scale with what you are looking for (20,000 + battles) but it is on par, if not larger than the MoM battles (where there was a stack limit that made even the largest battles, a fight of stacks)