Remember, it's all about perception. A big city doesn't really have to be that big, just big enough to get lost in.
I like to think in threes - some cultures even limit their counting numbers to one, two, and more than two, so there might be something neurological about it. One thing is a thing, or rather THE thing. Two things is a pair, a couple, obviously in a vaguely exclusive relationship with each other. Three things, though, that's just a bunch of a things, a crowd. Four is even bunchier, but three is enough.
So if you want your city to seem big, give it three or more sections. If you want it to seem small, just give it one section. Two sections means it's big enough for something interesting to happen, but nothing terribly complicated.
And by section I just mean any kind of geographic area that your brain registers as something different and distinct from the rest of the area, say the main road, the back alley, and the dingy part next to the tavern. If you really want the place to seem big, divide each of your three sections into three subsections, each with their own distinctive and memorable look.
For example, my Wassau city-area is just one 16x16 exterior, but I think it feels bigger than the other settlements in the module (Danaan Unvanquished, or just look up the Wassau pre-fab). It has roughly three areas, the Gnomish, Imperial, and Temple quarter. The Gnomish quarter has a long street with palms, a bunch of tenements clustered about a courtyard, and the Bagnio. The Imperial section has the forum, a massive Arsenal building, and the docks. The Temple quarter has a temple, a rich-people neighborhood, and a big impressible commercial building. You feel lost just reading about it, right?
The same works with people, too. For gameplay reasons, you want most of your townsfolk to be nameless, generic town walkers, so the player doesn't go around thinking they owe him a sidequest. But you can customize them with animations and props. So a simple village just has some villagers, all more or less the same. A small town has a blacksmith and a drunk and a merchant. A city, though, has multiple blacksmiths and drunks and merchants, each doing their own little thing. It's not really about numbers, its about showing the divisions and diversity organic to a larger settlement.