Manual partitioning (via Disk Utility on Mac, or via Disk Management on Windows) is generally a no-no. It messes with Drobo’s head, for lack of a better description.
No worries on being wordy - that description helps a lot, even for a not-so-Mac guy like me. Besides, I’m wordy too…
The way Drobo’s Thin Provisioning works is it’ll create a new volume once your available protected storage exceeds n x volume size, where n is the number of volumes you have.
So, for example, if you were to set a 4 TB volume size and had four 2 TB drives in your Drobo S, with Single-Disk Redundancy, you’d have 6 TB of storage (5.5 TiB*), and two virtual volumes (drive letters). Each volume could use up to 4 TB of storage, but doesn’t have to - you might have one at 4 TB, the other at 2 TB, or any mix in-between.
I don’t think there’s a way to force a new drive letter without the Smart Volume support that the DroboPro has.
But on Drobo and Drobo S…
If you were to set an appropriately-small volume size, you could start off with 3 volumes, but if you were to increase your available storage in the future, you may end up with more volumes.
You could add extra storage so you get up to 3 volumes of some size (ie, four 2 TB drives and use 2 TB volume size to get 3 volumes), then remove the drives you aren’t going to use, but that seems a little silly unless you have access to a number of large drives that you can erase and use temporarily.
If Airport Extreme lets you set different passwords for each volume, this would get you halfway there - you’d have different passwords for each volume, and even though all the data lives on the same Drobo, they’d be isolated from each other that way.
But it still doesn’t give you the space limit - any of the volumes could be filled up to whatever its maximum size is (or the maximum available protected storage, whichever is less).
You may want to consider a more-server-ish solution like a Mac Mini or something. WeGotServed is running an article on using OS X Lion Server as a home server - while it may be overkill for your needs, it might be a good read. I don’t know the full capabilities of OS X Lion Server, but it should give you more flexibility than the straight attach-to-router NAS. Granted, it’s also extra cost and learning time for you.