how densely populated would you expect the land surrounding Highgarden castle to be? are city-sized towns likely?

Well, the Reach is the most densely populated region of Westeros, so the most dense. I doubt there are any city-sized towns (i.e, as big as White Harbor’s 50k) because you’d think they would have gotten charters and become cities, or at least we would have heard of them. My guess is that you just have a lot of towns that are in the 10k range, more decentralized.