How big can a pulsejet be?
At the point where the structural rigidity needed to overcome the vibration of a single enormous pulse train becomes so heavy that the aircraft can't fly anymore, replace the single large pulsejet with several smaller ones, whose individual pulses are smaller.

Conversely, imagine replacing a car's V8 engine with a single-cylinder engine of the same power.  It would need much stronger engine mounts, flywheel, clutch, transmission, etc.  (Single-cylinder car engines weren't that rare a century ago; good luck finding one today.)

But the main reason that large pulse jets are rare is their low fuel efficiency.  The bigger the engine, the more that matters.

See also: What are the pros and cons of a pulsejet?