Once you research Organized Warfare, does army size matter at all? Is there a limit to the number of re-enforcing armies that I've never hit or something? It seems like a totally pointless stat with the exception of the ransacking calculation. Am I missing something?

Army size do matter after organized warfare for a number of reasons:

• Larger armies have reduced upkeep cost, romans in particular have a +1 army size as well as a -30% upkeep cost reduction to field large amount of units for low cost.
• During sieges, the army size of the attacker determine the number of potential siege weapon they can build and for the defender the number of militia they can use.
• In battles, mostly important for the defender is that armies a few tiles away will not start instantly deployed but have to be reinforced, which can be blocked by the attacker during their first turn, so larger armies may have a bit better survivability.
• As garrisons, only the largest army in that city will give stability, +5 stability per unit, so a larger army will give a bit more potential stability to your city.
• Larger air squadrons may have better survivability, like split the damage the aircrafts suffer more, but I have not gotten to test that, same may apply to units being bombarded by aircrafts or artillery, they seems to split the damage better.
• Larger armies mean potentially more army strength which impact things such as how quickly you can ransack, build outposts, chop wood and probably a few other things.
• It is a bit easier to move around fewer but larger armies, especially in terrain you have little space to move in. Also fewer but larger armies take less effort for the player to order.
• Number of militia drafted for militarist or horde units created for huns/mongols are based on army size.
• Probably various other things I have forgotten about or don't know.

Thanks for the largest army = stability bonus tip, I was wondering how that worked!

Your welcome.