In Medieval 2 the max number of units per stack is actually 20, not 16, and the max unit size is 150 on 'huge' setting. That makes 3000 maximum per side, so 1 vs 1 would be 6000 men.

In Empire we can expect to see the unit sizes stay the same or increase, so I would say 150 is most likely, and anywhere up to 200 is at least possible.

CA have, however, stated that they are not aiming to increase on the 10,000 men per battle limit so I wouldn't expect to see more than 150.

Hope that helps.