You really would think they would put that in the official FAQ, wouldn’t you? 
I’ve seen it mentioned a few times that the capacity is 22 litres (about 5 gallons).
There’s a good discussion on vessels here:
You can save your PC having to work hard by having an insulated container (you’ll see some are using camping coolers) and filling with hot water to begin with. You’ll also see that you can use two PC’s in one container (obviously, I’d put them at opposing ends of the container to maximize their effectiveness). Also having a lid that seals will minimize evaporation - saving you from having to top up your water (and cooling off the vessel when you do so).
Yes, displacement volume includes whatever you put into the vessel to cook. (it IS heating that)
Umm…there’s three dimensions to calculating volume. 