Balance shafts, Bill. I'm guessin' that kind'a like Archimedes with his quote about moving the earth if he had a lever big enough to do it, you could probably make as big a parallel twin as you'd like and with acceptable amounts of vibration, as long as you also included those balance shafts in the basic design.
The cases on the old vertically-split twins had no room for 'em, of course.
