I don't know about changing the city size in 4, but I do know that you could get a massive plot of land to have multiple cities, and the game sort of evolves into making multiple specialized cities iteratively.
Ex:
1. Make a balanced city, save it.
2. Remove all the residential areas out of the city, save it.
3. Plot 8 copies of that no-residential city around a new city.
4. Play that city and only have residential cities.
5. Once that city is maxed out, repeat the process by putting 8 of it (Or multiple layers) around a city and only do commercial/industrial in it.
Last time I played SimCity4 this is what I did and I ended up getting a city with 1.6 million people in it.