Assault on Trenth. City and surroundings

[Dain]

Hi guys,
I've started to map a city for a HARP adventure of a friend of mine.
First time I do such a thing digitally and with colour, so it's very time consuming. Still trying to figure out how to do things.

I've got a basic idea of what I want, but nothing written on stone. If you find something definitely wrong or that can be improved, please comment
You'll find a basic B/W sketch of where I'm heading and the beginning in colour.

There's one tricky question I'm thinking all the time. How do you choose the scale of the map? Do you follow a guide when starting a map?
When I started, I used specific settings to match a A4 good print (300ppi, 3508x2480 resol). The thing is that everything is quite small and wonder what will that be once printed.
I know that sometimes, the scale doesn't apply on buildings, but on lands, just because it make the buildings pop up and easier to see/ identify.
Do you guys calculate something prior to draw anything?

Cheers