Determining the size of a zone still eludes me. How do you determine the number and size of zones when you are in a combat situation in Dresden Files?
Zones represent ease of movement and the ability to manœuver. But also, remember that…
Zones serve the scene
Zones are thus a tool to use as you see fit to frame the scene the way you want. They're decidedly not about simulating a place "right" or anything like that.
Is this a sniper battle? There are several zones between the parties, and they're long, narrow bands. This gives lots of opportunity for ducking into cover and being shot at if someone (foolishly?) tries close the distance.
Is this a fistfight? Do you need more than one zone? Consider Harry duking it out with the gang of lycanthropes by the highway. There are a few zones there, but the existence of each is strictly for scene-framing purposes:
So there you see how the zones are really determined by the needs of the scene.
Rules of thumb
However, there are some useful rules of thumb if the needs of the scene don't answer the question of whether something is a zone or more than one zone:
However, zones are necessarily an abstraction and one layout for a particular scene won't necessarily be the right layout for another scene in the same location. The sniper-fight layout of banded zones wouldn't make sense for a fistfight in the same street. Neither would the layout of Harry's fight with the lycanthropes by the highway make sense for a magical duel that raged across the whole highway, stopping traffic – nobody cares about half-ditches then.
So, zones are determined by how they need to serve the action that's about to unfold.
The nice thing about zones though, is that they're very forgiving. You could lay out the zones for a fight in several different ways, and all of them will work. They'll influence the scene differently of course, but all of them will be interesting. You don't have to get them perfect.
Example zone layouts
A highschool gym during a basketball game
Victor Sells' beachhouse
Judging just from memory of how the fight and the shooting with Sells, the demon, and the scorpions went, I think these zones would suffice (though of course some other layout would also work):
Simply put, in all Fate 3rd Ed Games (of which DFRPG is one; also Spirit of the Century, Diaspora, and Starblazer Adventures), the size of the zones is story determined.
They're big enough to require shooting past, small enough to go HTH with anyone inside, and divided as needed for story purposes.
When I last played SOTC, the GM had us in several combats with very different sized zones.
Combat one: The hotel hall. Each hotel room was a zone. The halway itself was a 3 zones, each with 4 rooms off of it. Bathrooms were ignored for simplicity.
This gave us a good enough idea that he didn't actually DRAW the map, but simply described it. Numbering is mine, and the rooms aren't to scale.
The second example from the same campaign was the Queen Bee's lair - on an airfield in Norway...
Zone 1 Runway N (2,3)
Note that I used two different methods of drawing the maps, too...
You can take a map (like the hotel) and just divide off large rooms as needed, using rooms as zones premade for you...
Or, you can simply use numbers or labels, and draw connection lines. Either works fine.
And fate isn't the first game in print to use such a system. Crimson Cutlass did so back in the 80's. And all the classic Interactive Fiction works do so as well... both on computer and in gamebooks and solo-modules. It's highly flexible, and reinvented often because it's so powerful.
The answer to his is highly situational, because like most things in the Dresden Files... there is no true answer.
There really is only a cost for moving between zones, so you don't want to make zones so big that it seems strange that your players and NPCs can easily move within such a huge area. Take the example of having a Wizard's duel at a baseball arena.
If the whole playing field was one zone, that means that people could enter the zone from opposite ends and immediately get into close combat, or move through the zone extremely quickly. Now that might be an appropriate zone size if all my players are Wizards with super speed or Werewolves high on the light of the moon, but is likely far too big for mortals.
From there you attempt to determine what is appropriate and dramatically acceptable. If they are going to get into close fighting, maybe I make the area around each of the bases a zone, as well as having the benches for each team being their own zones. Now you have a series of zones that are appropriate for the situation at hand.
If I am instead having a big chase scene where they are in cars and fighting it out while they drive through the streets, the zones of course need to be bigger. Then it might be appropriate to have every "street" be its own zone as they maneuver through the city.
When it comes to Dresden Files, I just go with instinct. There is no right way of doing it and there really is no wrong way either. You will quickly know if what you made is too big or too little and can adjust accordingly.