The first thing I want to cover is size!
I think a good range is 4 to 10 unique, enterable buildings for the absolute smallest and biggest towns/cities in the region, unless it's something that really warrants an exception, like Lumiose where you access different segments at different parts of the game (although even that was way overdone!). As a player who likes to try to talk to every NPC/enter every building, this is nice because it stays fun and enjoyable rather than becoming a tedious chore to explore.
There are ways other than making a lot of buildings to keep a city feeling big - for example, Nimbasa City in B2W2 has ten buildings, Pokémon Center and both Gyms included (eleven if the Ferris Wheel counts, but it's not something you walk around in like the rest). Even so, they're spaced apart really effectively to convey the feeling of a big city.
> First, Nimbasa is split cleanly into sections: the main city with the Pokémon Center, three buildings and the Battle Subway; the amusement park with two Gyms; and the straight street to the north with the three biggest buildings. I really liked this, because it spaced everything out enough not to feel cramped, and you knew where things were, and the aesthetic had a ton of character.
> The second thing is that, from an in-world perspective, all of those sections are very logical, which gives the city character, shows how and why things are there, and makes it easy to remember where things are. The amusement park has the old Gym, which is a rollercoaster, and a Ferris Wheel, and then in B2W2 the second Gym was added there because that's where the old Gym was, so it's logical from an in-world perspective. Meanwhile, on the straight street, the three buildings are for sports and musicals; people go there to spectate events. Finally, the residential areas and Pokémon Center are all placed together in the inner city, and the subway station is near the houses for convenience.
> Even though the amusement park only had two of Nimbasa's buildings, the NPCs there, the Pokémon balloons and the general aesthetic did just enough to capture the feeling of an amusement park and feel like its own place. Your cities will feel samey and bland if you only rely on buildings to cover the bare minimum; you should have unique points of interest that aren't buildings and a distinct aesthetic, theme and purpose for everything if you want your city to feel alive.
> You actually also had to go to every part of the city and pass everything by at some point, so it didn't feel like you were going way off-course and making an unnecessary trip if you decided to explore; it was already where you were going. You enter from the south, sure, but then you have the Gym in the amusement park so you have to go there (also N in BW), the Pokémon Center in the bottom left corner so you have to go there, the Battle Subway right in between those so you have to pass it by, the Pokémon Musical building had Bianca's story event in BW and the middle of that street had a Plasma fight in B2W2, and that itself was on the right side of the street so you had to pass through the whole street to get to the exit on the left. There was no part of the city you didn't have reason to touch in-game.
So going off of Nimbasa, if you want your city to feel big but still alive, and you want players to enjoy exploring every nook and cranny, you have to divide it up cleanly and organize it well so players always have a point of reference and won't get lost, you have to pay attention to the themes and aesthetics of every part of it instead of making it all look the same, and if you really want it to flow naturally and to make players see it all, you should design it so they actually have to touch every part of the city at some point - entering the buildings can be optional, obviously, but seeing them from the outside is important and an entire row of buildings shouldn't be possible to completely miss!
Another thing: I don't really like doorless buildings that only exist to take up space. Actually, while we're using Unova as an example... Opelucid City is designed as a version-exclusive, and it just happens that one version shows what I like and one shows what I don't! In all versions, there are seven enterable buildings in Opelucid City. Beyond this, there exists in all versions a grid of buildings surrounding the city that serve to extend the background.
In White and White 2, this grid is entirely cut off anyway - there are trees, bushes and such that keep you from reaching any of these buildings to begin with, so it's conceivable that people live in them and you just can't reach them - they're not a part of your adventure, but they could still be a part of the world. Throughout Opelucid in these versions, there are also more trees and some elevation changes to guide you along a designated path - trees and elevation changes. These fit well with the world and keep you where you're supposed to go without taking you out of the experience and making you think "wait, why is this here?"
In Black and Black 2, you actually can reach a hecking ton of doorless buildings. Of that grid I mentioned: in the White version of the city, all four sides are cut off anyway, but in the Black version, you can touch one of the buildings specifically from that grid on the left side and all of them on the right side, making you wonder why you can't enter them. EDIT: Correction! Actually, that grid is inaccessible in both versions. I was looking at a low-quality map and didn't notice the gate on those sides, sorry!!! And beyond that grid that's in both versions, there are a further six buildings that are used solely as blocks and, again, have no doors or function in the world. It's significantly more immersion-breaking than in the White version, because while the White version skillfully arranged natural roadblocks that fit in and made sense to keep you from reaching that outer grid, the Black version has two sides of the grid and a further six buildings that are fully possible to reach and touch and should by all means be interactable because they would logically serve a purpose in the world if they were constructed, but they have no doors and... well, no, serve no purpose in the world. XP
To put that in perspective, in both the Black and White versions of Opelucid City, there are seven buildings you can enter; in the White version, there are some doorless buildings, but the player cannot reach them and they're not awkwardly placed; and yet, in the Black version only, there are eleven EDIT: nope, still six - see above! Six is still a lot worse than the zero in White/White 2, though, haha doorless buildings that the player can reach and touch and that serve no purpose in the world. I think that the White version of Opelucid City handled them just right - you could see them and they made the city feel bigger, but you couldn't reach them and they didn't break immersion while the city was big enough with the seven buildings you could reach. I think what's most important here is that, as a player, I feel like the White version of the city felt bigger because of the unreachable doorless buildings, but the Black version felt smaller because of the reachable doorless buildings - in White, it felt like I had access to everything I needed and I could reach everything in that part of Opelucid that I could reach, while in Black, even with the same enterable buildings, the doorless ones were too intrusive and I felt like I was being robbed, whether of space that could be used for walking or of buildings that could have been enterable but weren't.
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