I like Jody's rule, just name the displacement and call it a day. However, I would be reluctant to include Wankels because it's relatively difficult to compare their displacement to piston engines. That said, I really like the elegant simplicity of the technology; they remind me a lot of two-strokes in that they're traditionally known for being small, having few moving parts, generating a lot of power, making a sort of pissed-off buzzing scream, liking lots of gas, and being relatively neglected in their development.