You can increase upto 216.000 by changing the buildins happines and population bonuses , I have no idea but it does not pass 216 000 maybe game devolopers limited it like that.
if you mod just change population and happines values in descr.buildings...
I mostly play in BI with this. I have 12 000 000 population in Eastern roman empire, and of course I have more than 100 000 soldier. with 40 provinces. you can do just the same and of course can mod descr.unit to make them cheaper...