Cluster baser fluid catalytic cracking (FCC) riser models are widely used for studying the effect of operating conditions on the products yields. Cluster size is an important parameter that influences the particle residence time, heat and mass transfer coefficients, and product yields in the FCC riser. In most cluster based models a constant cluster size (6mm) is used. In the present work, various correlations for the prediction of cluster size as a function of local flow conditions are compared. The results show significant variation in conversion, and yields of gasoline and coke for these correlations. Coke yield prediction is most influenced by the cluster size.