I wouldn't say they are 2 stroke, or 4 stroke, as there are 3 parts to the "stroke"
Not entirely sure about this one.................
Capacity of any engine is combustion x cylinders.
0.66 x 2 = 1.3L. It is the number of of combustions per revolution that changes this compared to a piston engine.
A piston engine only fires (combusts) half of the cylinders per revolution, the rotary uses both combustion chambers per revolution. It's easier to double rotaries capacity in this sense instead of halving pistons, when comparing capacity
I've seen some 1000+hp 13B's. Thats pretty good power. Even for "3.9L"
2 Spark plugs is for pollution, it burns the excess fuel/air mixture that doesnt burn from the first spark, an inefficiency.
Kinda like the Z-series motor in the nissan family
A standard box for a 13B isnt that big. But when they quite often match up a celica/supra 5sp or alike to handle the power, it is rather big.