Calibrating the output of a smaller source


I am using GATE(which uses Geant4) to do MC studies on dosimetric output. I am using a cylindrical cobalt source at 80 cm SAD to measure the PDD in a water phantom and dose at depth of 10 cm.

I now want to simulate a smaller source (say, r/2 and h/2) and compare the dosimetric output at a depth of 10 cm. Besides the geometry, I see that I am able to control the number of particles and time of the simulation. What would be the best way to change these two parameters to mimic the lower output from a smaller source? Or is there any other parameter that can be changed to mimic a smaller source?


P.S.: In case it was not clear, I am trying to calculate the output factor of the smaller source keeping the larger source as reference.