Could I suggest looking at the following data for population of *metropolitan* areas within Europe - probably gives a better idea as to potential number of passengers compared to those living purely within a city's official boundaries
Note how Paris is 11 million, Athens is reported as being around 3.75 million, and Manchester at 2.5 million
List of metropolitan areas in Europe by population - Wikipedia, the free encyclopedia
Alternaitvely for urban areas in the EU (EU LCCs would need a good commercial reason to want to fly to places like Russia), look at:
Largest urban areas of the European Union - Wikipedia, the free encyclopedia