The problem here is that wind erosion is going to be pretty terrific. Those "clumps of dirt" won't last long against hurricane force winds. Remember that a human body falling at terminal velocity falls at about 120 MPH, and dirt plus rocks certainly won't fall slower; in fact, with higher density, they'll fall faster.
So either the city inhabitants have coated the bottom with something to keep things from being blown away, or else they have to constantly replenish what gets eroded away. I can see a story idea here, with workers having to constantly work on the canvas (or whatever) covered by netting, which is needed to keep the soil from being blown away.
For larger structures, the question of fluid dynamics does come up. But equally important is whatever provides the structure keeping these falling "islands" together. Steel, aluminum etc. simply can't hold an extremely large structure together. The cube-square law places severe limits on the ability of large structures to support their own weight. (And these islands are not actually in "free fall" as the term is used in physics. They are falling at terminal velocity, which means they do have weight. The people walking around on these island will not float off; they'll feel weight.)
Frankly, unless you're talking about tiny islands no bigger than a skyscraper -- which would be the approximate limit of size using concrete and steel, or possibly even smaller if constantly subjected to hurricane force winds -- you're looking in the wrong place for a hard science answer here. If you want something that far removed from everyday reality, either assume the laws of physics are radically different (but then, that means anything living has to be far different from life as we know it, so nothing like human beings in the story), or else magic is being used to hold the islands together.
Another possibility is to use "exotic matter" with an arbitrarily high tensile strength, like /scrith/ in the RINGWORLD series. But that's near-magical science fiction, not real science. /Scrith/ has a structural strength on the order of the strong nuclear force. With a material that strong holding things together, you can make the islands as big as you want... or limited only by how much of the exotic matter the inhabitants can get or make. If you go that route, I suggest a honeycomb structure of the exotic matter (since it's that strong, the walls of the honeycomb can be extremely thin... even thinner than aluminum foil) with the voids filled with normal matter, or even empty space to make the structure lighter. The exotic matter could also be used for (amazingly thin) structural beams, arches etc. to support buildings on the islands.
Of course, if it's exotic matter, you can choose any arbitrarily high structural strength you want. It doesn't have to be as strong as /scrith/, but it should be stronger than carbon nanotubes... which in theory can form structures 30 to 100 times stronger than steel, but in practice don't seem to form structures with long-term stability. In other words, carbon nanotubes break down at the molecular level after a fairly short period of time, so apparently aren't suitable for building things.
"The aim of the city is for it to have a lower terminal velocity than a person, ideally, hence the name!"
Well then, you need to rethink the idea of the cities being composed of a random collection of dirt clumps, airplanes, and random metal junk. They will have to either be structures built with a density less than the human body -- a lighter overall weight per unit of volume -- or else they'll have to have something like giant literal parachutes slowing their passage through the air... and how long would those last if made of ordinary silk and nylon ropes? Or even kevlar? Again, this is suggesting exotic materials.
Even if they are held up by literal parachutes, there's still the problem of holding the entire structure together, and the severe limitations to size of using ordinary materials (for example, steel or aluminum I-beams).
The simplest solution would be to use the "golf tee" overall shape for the city suggested by user16295, constructed of a lightweight honeycomb of some material stronger than what we can make with current technology. Assume much of the "tee" is empty space (or even vacuum) surrounded by honeycomb, with only the surface inhabited, furnished with soil to grow crops for food, etc. (I hope they get plenty of rain to provide water?)
You might consider investigating the theoretical properties of carbon nanotubes, but assume that some method of producing large structures made out of them, plus some method of stabilizing them on the molecular level has been discovered... which I suppose isn't impossible in theory.
"Actually: just thought some more about my last point: they'll be up at a full g, won't they?"
EDIT: I see I made an error in what I posted below. I'll leave it there for purposes of illustration.
No, you were right: A fraction of a gee, assuming the entire space is in a 1-gee field. That's 32 feet per second per second of acceleration for something on the ground. If, for example, the parachute city is falling at 10 feet per second, then that's (32-10=) 22 feet per second, which means 22/32 = 0.6875 g for those standing on the ground of a parachute city.
EDIT: My error above was confusing velocity with acceleration. No, those standing on the ground on the parachute city will be subject to 1 gee, just as those standing on the surface of the plant. They would only experience lower gravity if the parachute city was under constant acceleration. Since it's falling at a fixed (terminal) velocity, it's not accelerating. [end of edit]
But why would this endless column of air have a one-gee gravity field? This is obviously not Earth! Since you're world-building, you can arbitrarily choose any gravity level you like. And if the outside gravity field is lower, well then you can make the structure larger without using exotic matter. Lower weight means you can use weaker materials to hold the structure together, altho if the winds are as terrific as you suggest, the inhabitants would still have a severe problem holding things together for long.
Groups of objects held together by something flexible? Well now, that's something that might actually be possible given current building materials. Kevlar is your best bet, but your original question suggested that all that's available is some random debris. If they have to rely on ropes woven out of grasses or reeds, those structures are gonna have to be pretty small!
"If you can expand your answer up with any numbers to back up the 'no bigger than a skyscraper' assertion..."
Well, it depends on how flexible the overall structure is. Bridges can be bigger than skyscrapers, partly because they can flex and partly because they're more horizontal than vertical... so they can be supported by multiple piers rather than having the entire weight of the structure supported by the bottom floor. (If the weight is hanging from parachutes, the entire weight has to be supported by the framework the parachutes are attached to. But even so, a bridge might be a better analogy.) However, think of the behavior of a flexible bridge in hurricane conditions. It won't last long, will it? So that's why I think the skyscraper analogy is closer to what would actually survive under such conditions.
Sorry, I'm not gonna do math here. I don't do math for fun, and you haven't provided enough specifications to be able to give any hard numbers to it anyway. We don't know what the turbulence is, and we don't know how fast the parachute city is falling. And altho you've specified a one-gee gravity field, there doesn't seem to be any good reason to impose that limitation. Choose a lower gravity, things become easier for the inhabitants to build stable structures, probably the turbulence is reduced, and it becomes easier to fly from one structure to another. If it really was a one-gee field with everybody falling along with some random debris, it's almost impossible to believe that they could actually organize and build anything before starving to death.
BTW -- I suggest you read Larry Niven's THE INTEGRAL TREES for some ideas about humans living in free-fall... and on structures which are "falling through the sky" but nevertheless have portions of the structure where you feel weight.