Its not a question of map size. We have 199 cities and RTW doesnt allow more. No mod can have more cities then EB have. Thats it.
The only way would be to cut out east postion of map (make it smaller) and use this delated cities to create new cities somewhere else. But this is not an option.
Illyrian serteinly is not a good choice.
Half of its land is under Epirothes rule when mod starts and we have already many faction in that region. Its owfoul choice. Aedui/Arveni are much better more historicly importent and more game wise interesting choice iMO.