From a game design point of view, what are the implications of using realistic distances in an open world map?

I’m starting an open world game and the plan is to have a single "open world" map that should contain a medieval citadel (1.4 km/2 – taking as example Palmanova in Italy) and some countryside around it.

The plan is to delimit the map with a river and some hills.

What I realized so far is that most open-world games take a specific approach when it comes to size their areas, it’s unlikely you’ll ever find a realistically sized citadel, they will tend to be all pretty small, and it will take no more than a few minutes to run from one side to the other of it.

An example that comes to mind is The Witcher 3, the Novigrad/Velen map is 15.2 km2 but the city is approx 0.25km2, which is a lot smaller than what I plan to build for my map.

To walk from one side to the other of my citadel the player should take around 15 minutes (according to Google Maps), while, as I said, in most games you can run through a city in a matter of 1 or 2 minutes.

Keep in mind there will be no vehicles and I’d still like to have the countryside take most of the map area to keep everything proportioned and realistic.

The question is, is it a good idea to build such a "large" citadel? What gameplay issues could I face? How should I handle the fact that the countryside will end up being extremely huge (and empty?) if I want to keep it properly proportioned with the citadel?