Smaller units win battles. Larger units win wars.
I will have to disagree. Large units lose more when they panic and run. If you are commanding a unit as large as 200, you can lose 40 men from just panicking. Might even lose 40 more when you rally them.
Smaller units don't suffer the same way. Also, they are more likely to retreat than scatter meaning you may end up wasting hits on a small unit that you do not want to attack. Having more unit also means for melee units, your hits get spread out and you fail to finish your target. As for ranged units, it is even worse since you have a chance of attacking a single digit unit when you could be hitting a larger unit since you can only attack one unit per round.
Single unit may last longer but more unit wins you battles. Winning more battles and being able to sustain your advantage will win you the war not large units.