I guess it'll be the same as in rome. By default they aren't going to be huge, but if you and your comp can handle it, you can set them to huge.

That's what I love for example about the Age of Empires II... A simple RTS, designed for armies of about 30... but as computers got better and better, today you can play AoE II with big armies, almost like in Cossacks. That's why the game doesn't grow old for me.