I have created this small table, which I use to determine the size of a city based on the actual population. It is based on a formula I found on civfanatics and how Civ actually calculates the population totals.
NOTE: As a further variation I use the actual city population / 1,000 as the starting cultural level as well.
How to calculate population points for cities. 2016-10-05
How to calculate population points for cities.