If your drone is like mine, from 2200rpms to around 2700rpms, 28-30" pipes, one per site, should do the trick. Your drone frequency should be close to mine.
I used a mic to record the drone and then analyzed it with Audacity. Drone occurred at 125hz-ish. My pipes are 28" and its dead silent from 2300rpms to 2700rpms and no drone above or below.
Pics are in the intake/exhaust thread I posted about recently near the last posts.
LINK