In the options tick 'advanced' this brings up a bunch of previously hidden options.
The one to change is 'unit size'.
'Large' is 80 & 'huge' is 160.
Huge battles are awesome but can bring a powerful PC to its knees & also can give even bigger problems for pathfinding & unit placement.
I find large unit size to be best on average.

In Shogun & Medieval however, I used to almost exclusively use huge.