Rotaries are neither a 2 stroke or a 4 stroke, I'm sorry, but they do not have any pistons, so they are neither. There is no way around that. They are however MOST SIMILAR to a 2 stroke piston engine in how they operate.
A 4 stroke engine completes it's combustion cycle in 4 strokes of the piston - the classic suck squeeze bang blow.
A 2 stroke engine completes it's combustion cycle in 2 strokes of the piston - more or less suck-squeeze bang-blow (grossly innacurate simplification )
I don't even want to try and classify a rotary, but they are in fact neither. They are similar to a 2 stroke in the way they operate, using ports not valves, and that a single rotor is performing multiple parts of the combustion process at once.
On the other hand, they do perform a classic 4 stroke cycle in the form of suck squeese bang blow, on each face of the rotor.
2 stroke / 4 stroke, irrelevant, they are neither.
RPM, thats a tricky one. I tend to agree that the output shaft is the most appropriate way to measure RPM, but if someone were to put a 3:1 gear inside a piston engine to triple the output shaft velocity, where would you then measure it?
When you look at RPM in regards to the forces acting on the parts of the motor, well then it is really the 3000rpm the rotors themselves are doing compared to the ~6000 or whatever the pistons are doing. But then, a piston needs to actually stop and change direction, while a rotor just keeps spinning the same way, resulting in far less force anyway. But this is one of the ingenious things in the rotory design.
Capacity again, a tricky one. I personally would call it a 3.9L, but with the caveat that it compares more or less to a 4 stroke piston engine of 2/3 that. But I can see the points of the other arguments.
Basically it is very hard to compare piston to rotary motors as they are so different.
I am not a rotary fan boy, I love all types of engines, 2 stroke, 4 stroke, rotary, radial, jet. If it burns fossil fuel and moves stuff around, I like it.