1) open finder and navigate to your "Library" folder
2) go to your "Application Support" folder then the "Feral Interactive folder"
3) open "Napoleon Total War" then "AppData" and finally "Scripts"
4) open preferences.script.txt with TextEdit or any other text software
5) on the 10th line you should see "Campaign_Unit_Multiplier 1.00 (this number varies based on unit size setting in game)
6) 1.00 = largest standard unit size, 2.0 means 2x the normal size (so if there are normally 120 people in a unit, there will now be 240) so change this to the desirable amount. you can use 1.5 or even 0.1 if you want, but be sure that there are 2 digits with a decimal inbetween or it may not work.

careful! i have a 27" iMac and even i get lag at times with 2x (on max settings) use at your own risk! i am not responsible for your game or save file becoming unusable. test this in custom battle first to see how your computer handles it.