We investigate a recently introduced seismic fault model where granular particles simulate fault gouge, performing a detailed analysis of the size distribution of slipping events. We show that the model reproduces the Gutenberg-Richter law characterising real seismic occurrence, independently of model parameters. The effect of system size, elastic constant of the external drive, thickness of the gouge, frictional and mechanical properties of the particles are considered. The distribution is also characterised by a bump at large slips, whose characteristic size is solely controlled by the ratio of the drive elastic constant and the system size. Large slips become less probable in the absence of fault gouge and tend to disappear for stiff drives.