I would point out that the effective length of his manifold includes the throttle body itself since each barrel serves only one rotor.   It's not as short as it might appear at first glance.   44 mm is the right size in this application IMO.
Tracy  

That is true. The effective length is about 10-13” out to ‘air manifold’, depending on which port.  Distance to the throttle plates is about 4” less. There is also smooth transition from the air manifold, where the air is already accelerating, to the TB. The idea was easy breathing, and relatively high flow velocity.  I have no idea what the pulses are doing.

Al