If two rocks of different sizes start sliding at the same location, the larger rock should not have a longer runout distance. This is because Sliding in RocFall is modeled with Amonton-Coulomb friction. Amonton's first law states that the frictional force is proportional to the applied load. While heavier rocks have more momentum, they also produce a larger frictional force, in a ratio proportional to the mass, making them decelerate at the same rate.
Since RocFall uses a lump mass model, the rocks technically have no size, and are infinitely small point for most purposes. The size of the rock, as described by the density and mass, is only used for rotational velocity calculations during impact, since a point cannot have a rotational velocity.