Settlement population limits ..

[Horseman]

Thats a good point - if you use hidden resources to say limit a settlement to a Proconsuls palace then put in that the building reduces population growth by as high as you can/want again using requires hidden resource to mak ethis effect only in the settlements you want to limit you can cap the pop growth (except disbanding/slaves) You can make the minus quite high because AFAIK a negative to pop growth will never take it into the minus's

[Squid]

You can sort of do it with scripting, you can write a script that removed population if it's over a certain level, now this won't hard cap it at a number but will keep it close.