I'm currently going thru and getting the most reliable and recent population estimates for all countries. Most of this data comes from citypopulation.de which is a very reliable source. However, city/town/etc definitions are different by country, so it's not really even possible to get a standard population for a 'city' across all countries. But in almost all cases, a city's population is the city proper, not the urban area/agglomeration. Look for population quizzes to be updated as each country is brought up to date. Many countries won't be changing as there simply isn't new data available yet.