The maximum size of the spawn zone is 6 plots in every direction from starting plot. Radius starts small and increases by era until Medieval.

The spawn zone setup menu option determines how this area is searched. "Classic" chooses one city per Civ over entire area. "Iterative" chooses any cities that can be reached (can't cross seaways or mountain ranges that bisect spawn zones). "All cities" will select every single city in the area.

Next choose to enable "Convert Cities" mode. When it's disabled, cities only revolt to free cities. When active, cities can revolt to players (technically destroyed and recreated, so it's a different city from game's perspective, like original owner or grievances). The first city converted, the capital, will ignore the revolt rules -- unless it's a player's final city.

Revolt rules are as follows:
1. Not original capital or final city
2. No established governor in city

"Convert Any City" option ignores all rules and converts everything!