Wow, I really like all what I'm reading here about this new POPULOUS system! It seems like a major revolution - great work!
I don't want to go crazy here, but since you are "playing" with numbers... and there are a LOT of players here who would like to see more or less realistic unit sizes.... Would it be reasonably easy to add some kind of "alternative unit size modifier" or "presentation mechanic" that doesn't actually modify the unit size but only pretends to do so? I know this seems like gibberish, but to play with realistic unit sizes is obviously difficult, because the battle engine is not able to represent them. What I suggest is that a unit of 160 (ultra unit size)/ 80 (medium unit size) etc. will always be represented and calculated as 480 or 500, especially in the battle outcome... The calculations of your new populous system would also use this number of 480 or 500 rather than the 160 (ultra), 120 (large), 80 (medium), or 56 (small)... I remember that in Rome Total War there were some issues with population when playing with higher unit sizes, but with this mechanic it would ALWAYS be the same system no matter what.
In other words: if you actually count the men of a unit in battle they will have the same number as in vanilla (e.g. 160) or whatever mod you use with adjusted unit sizes, but in the text (battle outcome, populous system, unit info, army info) a cohort has more like the actual number of a cohort - 480 or 500 and all calculations will be made with this number and not the 160.
If on the battlefield 1 man dies in the battle outcome this wouldn't give:
killed: 1 - left:159,
but instead something like:
killed: 3 - left:477 or 497
This way "in appearance" we could have armies with up till 10000 soldiers (with vanilla 20 units/army) which allows for a real legion + auxilia, extraordinarii, name it up under one general.
I think the only really necessary visible changes this needs as far as the vanilla game is concerned would be a different number/ calculation in the battle outcome and the army (& unit) info. As it is possible to turn off the unit numbers in the unit cards already, that aspect may be of less importance.
Maybe an interesting idea for a future add-on someday when you have more time?... If the idea appeals to you at all, that is...
Anyway, your population system looks awesome, can't wait to test it!^^