I'd think the number and size would depend upon how quickly you want to ramp up - combined with the types/sizes of fishes you plan on getting.
TTM requires two tanks, but only one will really be in use at a time. The other will be drying, basically. And TTM only lasts 12+ days. The rest of the QT time is spent in a single tank. So as long as you stagger your new-fish batches by 14+ days, you could run two TTM cycles with three tanks (eg; not four). Along the same lines, you would be able to respond to an ich outbreak in the DT in the same fashion. You can also use your TTM tanks as hospital tanks when/as needed - so long as you're not in the middle of a TTM cycle. That's a chance I've been willing to take so far, personally. If absolutely critical (eg; needed a hospital tank while in the middle of a TTM cycle), I could always go pick up a 10gal nearly anywhere for cheap.
So, in short - I'd think two QT/hospital tanks is enough, while three covers every need.
As far as size, that depends upon what you're buying - and how much at once. I limit myself to young (read: small) fish as it gives me more time to watch them grow. I also do batches of limited size (<4, typically). As such, I've been able to get away with 10gal tanks so far. I've not done any tangs yet, so I may need more QT space when I go to introduce 4 of them into the tank at the same time. But even with that, I'm glad I have the 10gal tanks for most fishes as I use less water to fill those up. TTM is great (IMO), but it's biggest downside is that it puts a lot of water (and salt) down the drain. As such, keeping the volume as low as possible makes the most sense to me.
Also as a reminder; you'll want the QT tanks 10ft+ away from each other and your DT to prevent aerosol transfer. It's unclear to me whether a good cover on the tank (obviously keeping good air transfer) could reduce this distance, but that might be an option to help reduce the footprint required...
