Scale and Size Questions for big city mapping project

[Eld]

Hi everyone!

I hope this is the right subforum for my thread. If not the moderators may feel free to put it in the right place.

First off a short introduction for you to see my point of view.

The Project
For a long time I plan on mapping the city of Renaliac, the queen of cities. This is a huge task to me that will take years to finish, yet I desire to do it.
Renaliac is the most beloved city of mine of my entire fantasy world Nerrac. It's the economical, cultural, religious and political center of a big part of the continent Adaichin and has about 1 million citizens. The city is more than 3500 years old and is accustomed to be a capital.

The Method
The project will take so long as I don't simply plan to map the modern city. Primarily I'm inventing and exploring the world, mapping it is secondary and just one tool to show it as writing is another tool for me. That means that I want to develop the hole urban history of the city and will start at the very beginning, more than 3000 years ago. This method has the benefit of providing me with lots of knowledge about the city and I come to know about the whole structural and architectural development of the city. I hope this finally results in a map of the modern city that is much more plausible and authentic than it would be if I started with the modern map. Plus I will already have maps for important points of time in history and don't have to create them after I got the map of the modern city.

The Problems
Facts: I use Gimp 2.8.4 and the city covers an area of roughly 30km² plus some surrounding area.

1) Scale and Size. I'm not sure which scale and what size I should choose for the map. I want it to be easily printable afterwards, so the dpi-resolution should be rather good. Also would it be nice if the scale in meters/pixel could easily be calculated into cm/cms on the printed map.
At first I thought of 1m per pixel and a resolution of 381dpi (what becomes 150px/cm and provides for a nice 1:15 000 scale at the printed version). This would give the map a size of about 6-7k x 6-7k pixels. However, I looked at some great city maps here on the Guild and it seems to me that 1m/px would be rather pixelated, while other maps have neat contour lines for their house blocks and stuff.

What would you suggest in view of the map area and size needed for a poster (A2 which is about 42x59cm).

2) Layers for the historical maps of the city.
I plan to give each historical map their own layer. So the layer with the terrain will suffice for all and the city buildings will get their own layers for each point in the timeline, e.g. one in -300, one in 29, 250 and so on till 3167, the current year in my world. I'll need about 17 to 20 such points to show the development of the city. The advantage is that I can simply duplicate the older layer and have a pattern for the later layer. That guarantees that buildings which endure will have the exact same shape on each layer while changes could be achieved by erasing.

Yet I'm not sure whether the file would get to big for Gimp if I have about 20 to 40 layers on such a big image. (at least 6k x 6k px, probably much bigger)

Finally I'll show you a first draft of the terrain that Renaliac lies in (in red the area of the city).
Entwurf.jpg
And this example shows in which style I plan to map the city. It's the city of Berin, a small capital of a small state with about 70k citizens. However, I hope to develop my style, too.
Berin 3167 Beschriftet.jpg

Thanks for reading all that text and for any help, suggestions, ideas and so on in advance!