Historical army sizes

Well this is not necessarily about the game itself but (in my case) a lack of information...as an example at the Battle of Pharsalus Pompeii has about 45000 infantry and 6000 cavalry as opposed to Caesar's army of 22000 infantry and 1000 cavalry. Now I suppose that such an army would be formed of a number of legions now my question is this: In those times did the fate of a country\faction was usually decided by means of a few large battles (with such numbers as mentioned above) or by means of smaller but more numerous battles (with armies close to the number depicted by the game 3000-4000 men strong -lets call it a skirmish) and did an army that large was the full military power of that faction on that given moment in other words did a faction committed all its forces into a decisive battle?

And finally one question about the game itself especially the AI if the first case (large battles) is more historically accurate can the team tweak the AI as to use coordinated attacks of a number of full stacked armies in order to somehow simulate large sized battles and the diplomacy to easier accept a ceasefire and\or negotiations if defeated?

Hope I didn't asked to much here :D

All the Best

Civil wars more often see either a quick resolution by betrayal or large armies gathered and a decisive battle or two. Punic wars and other wars of large states usually saw numerous battles and skirmishes. There might be still a decisive battle but before that can happen usually one side has to secure a dominant position to bring that decisive battle and so its hard to say. The size of battles in MTW2 even maxed unit sizes are probably between a skirmish and a large decisive battle. However many times the states fighting were relatively small and did not have the huge amount of men to call up that large states did so a battle of 4,000 on each side would have been decisive with perhaps a few further skirmishes to enforce the victory. There are so many exceptions though...

The most famous and recorded battles are usually larger but a campaign is called that for a reason. Usually to force an enemy into a decisive battle there were already many smaller engagements fought so... not sure if you can force MTW2 AI to invade in a coordinated manner without simply making it have huge bonus that despite it wasting 50% of its forces in weird marches the wrong direction it still is able to send a large amount on the offensive in the same direction.

It depends on the people and what you are doing. Certain people could raise absolutely ridiculous armies from the lower classes for defensive battles.