It is not possible for the size of a radiation emitter to be greater than the wavelength of the pulse emitted.

Pulse Widths are found to be 10 to 20 millisecs

The radius of the emitter would have to be  c (20 / 1000 )

Or 300,000 *  0.02 = 600) kms  (Approximately)

6000 KM = Approximately the size of a small planet