Air vents in an apartment building

There is a lot of variation in how air vents are designed. To a large degree it would depend on the size and design of the building. If you want someone to use them for access, then do it. You will be writing a work of fiction, won't you? If the building is large and multi-storied, then you should be safe to say that they could be used for access. There might be places where someone could walk through the ducts without any trouble.

In my apartment building, there is no way someone can crawl through a vent into an apartment. These vents are so small that you could barely fit a hand through, unless you have someone who can shrink extremely small (Thumbelina sized) or turn into mist.

On the other hand, you might ask a builder or architect to see if what you're looking for is possible. It might be in a large city.

I've never seen room vents small enough for a person to crawl through. The only way that works is if the building is very large, then sometimes the main AC vents will be big enough but the branches leading to specific areas will still be too small.

Many systems will draw in air from the roof so access wouldn't be a problem.

If you're looking for access for a sneaky person though, older buildings often have access points where electrical, plumbing and ventilation all travel through spaces inside the building. Many of those are big enough to crawl through, though in an apartment the only exit would be into a corridor, not usually into a suite. The Matrix used a space like that at one point (escaping the baddies on Neo's first trip back to the cyber-world).

Also depends on the age of the building. Some of the ones in our area have no air conditioning at all (yes, seriously--and in a hot, humid place).

The one I lived in has wall units under the window (hitting a few inches under the window sill). Air blows out from the units. Each one is connected to the one below/above it, and a big system in the basement. No one is going to do a Voyage to the Bottom of the Sea to it (a show well-known for people using the air vents to crawl around).

Though, overall, I'd say no, not for an apartment building. The vents would need to be awfully large to accommodate a fully grown person, and I can't see a reason why they would need to be that big (and, besides, they may be intentionally small now so that people can't use it to break in). In all the places I've been--Army WWII barracks, apartments, houses, the vents have always been pretty small. Don't need a lot to get the air out into the place.