Hi There,
The tutorials here start with building a house and cover a lot of the building blocks you will need to build a city like creating cells, laying nav mesh, lighting, placing objects, etc. From my experience, learning the basics of modding first will help you avoid making large scale mistakes later in your city, so definitely a good place to start.
That said, once you can build one functional house you will be able to build more to make villages or even cities so modding many buildings isn’t that difficult; it’s a lot of repeating basic skills.
There are limits to how big you could make a city, obviously you’re going to have to be mindful of what’s already in game and where you could plant a large city among what’s there. The developers would likely create a new world space for such a city and force players to take a boat or portal to reach it to avoid competing with the existing game space. That’s certainly a technique I recommend if you want a really LARGE city.
The hard limits are difficult to precisely nail down. The more you add to the scene for the engine to render at once the more it will impact performance. If you have a powerful rig you might not notice but if your rig is more modest it might struggle to load a full city. One way to combat this would be to break your city into several cells with load doors between districts to reduce the load to sections.
Sounds like an ambitious mod, which is awesome, I would just recommend getting the basics down first to save you some headaches. Happy Modding!