Currently Im growing my cities as large as possible but Im not sure that is really optimal. There are quite a few problems with growing large cities: Getting happiness with monarchy don't work too well with pacifism meaning the saving on city maintainment is lost to unit upkeep. Getting health is also problematic with large cities and you can not afford losing food to unhealthiness. This mean you probably need to build infrastructure which cost production that could be otherwise spent on other stuff. Each city produce 2 food from just existing which mean each city can support basically a specialist for free. With mercantilism each city you have get another specialist for free. More cities with less population works better with slavery. Growing population get more and more expensive. Looking at the math a bunch of relative small cities that run 2-3 scientist can chain generate a great scientist every 4-8 turns for the 10 first great scientists assuming your run pacifism. Getting 4 to 6 surplus food is not all that difficult, a city start with 2 food already and to get 4 more you need 4 grassland or 2 foodplain farms.