The GEMM Database contains several different sets of materials which are designed to cover a range of different material sizes. Each one has a size distribution based around a median value, an example is shown below:
Particle Size Distibution, Median Size = 32mm
If you’re not sure which size distribution to choose, start with the largest and
give it a try in your application. It is much quicker to work with a test case
where the particles are too large than it is to work with a case where they are
too small because - with a large scale material - the particle count will be
lower and therefore the simulation will progress quickly. If the particles are
too small it can take a long time to fill a volume instead of progressing
with the simulation.
The Engineering team recommends keeping
the scale as large as possible initially, only when more detail is required
should the particle scale be reduced.