Increasing Unit Size
I know there are mods for increasing unit size, but if you want to have 10 thousand men in a unit, Then hear Me out. First go to your file, then drive, click on users then your username, go to AppData, Roaming, And you should find all total war games post Medieval 2. If you want to modify unit size. Click on the total war game you want to change. In this case Shogun 2. Click on the Shogun_2 file, Then go to Scripts. If you have mods on your game there will be a users.scripts. and preferences.scripts.

Click on preferences.scripts. After you do that search for the text line called "Campaign unit multiplier" . The original in 0.75. I set mine to 4.5 so my ashigaru has 900 men per unit instead of 200. You can go up to 20.00 if you want to murder your computer. Under that text line is also the naval fleet sie. Change that if you want to. Mine is 1.75