After some thinking, I got to the conclusion that a large city would be a bit overwhelming for my NPC-centric gaming style. I'd like to have something more personable on hand, maybe a port town, called Zabreba, with 2,500-5,000 residents. This will probably have a few nobles and merchants present, and about two inns, one small bookstore and one or two "magic" (i.e. ingredient) shops.
2,500-5,000 residents - still enough for very cool urban adventures IMHO, but much more personable than a large city.
It won't be a city-state per se. HOWEVER, the Kingdom of Zagadur would be quite weak and corrupt, so the King will rarely be able to intervene in local affairs, leaving the Mayor as the de-facto supreme authority in Zabreba.
It will still be an ice-age setting, with domesticated Mammoths used as draft and war animals, and huge beasts roaming the countryside, hardly held in check by the City watch in the lack of any real support by the King.