A cluster size of a perpendicular medium was calculated from the correlation between grains in M-H loops. The cluster size was affected by physical properties as follows. When grain size is small and exchange stiffness constant A is not zero, the cluster size increases with decreasing grain-size. The increment of the cluster size decreases with increasing grain size distribution. The cluster size divided by the average grain size (normalized cluster size) increases with an increasing intergranular exchange coupling constant he. The normalized cluster size can be estimated from the grain-size distribution and the intergranular exchange coupling constant.