Find the bit saying 'campaign_unit_multiplier 1;' and change the number. Obviously the number acts as a multiplier for the number of units. Eg, 1 would give 120 men, 2 would give 240 etc
Keep in mind the bigger the multiplier, the more powerful system you will need to handle so many units.
There are a few interesting bits and pieces in the file which can be changed, though mainly gfx related. Remember to back up before changing. Changes my revert after patching or adjustments with unit size options. If it keeps doing it, just set the file to read only.
After changing the multiplier, it might say under options in game that the unit size is small. Ignore that, it isnt small.
This increases unit sizes in both campaign and custom battles. No idea if it affect multiplayer,
And yes, i shamelessly ripped most of this from my Napoleon total war post since it applies to Shogun 2 as well.