Displacement is measured by the amount of air moved per cycle set, right? So, it's not a 3.9 liter, since per 1 rotor revolution, only 1 side of the rotor is used to move air, compress it, then combust it, and expel it. But one could theoretically argue that it's a 2.6 liter, if you compensate for the extra emptry stroke cycles done by the piston engines. For every up/down stroke, the crankshaft in a piston engine moves twice. Wankel motor completes its combustion cycle in a circle.
if you guys have a LOT of time to kill:
http://www.rx7club.com/showthread.php?s=&threadid=165579
there's a thread dedicated to discussing the displacement of the wankel engine. I'm just going by what the inventor, Felix Wankel, rated the engine at. Which is 654cc per rotor chamber (3 per rotor, only 1 used at a time) x 2 rotors, at 1308cc total.
On a side note, you can't measure the rotary by the crank rotation, because for 1 rotor revolution, the rotary makes 3 eccentric [crank] shaft revolutions. So when the tachometer on a rotary is showing 6k rpm, that's the speed of the crank shaft, the actual rotors are moving at 2k rpm