An independent approach for determining cluster sizes is proposed. This method is based on momentum transfer considerations and molecular dimensions. Cluster sizes of n-heptane and n-decane at several temperatures have been calculated by this approach. The results are compared with the average cluster size as obtained by the mass-action equilibrium approach of Ubbelohde and co-workers. Cluster volume fractions and average cluster sizes (calculated by the equilibrium approach) for the halogenated benzenes are also reported. The effect of decreasing symmetry of the molecules on the cluster volume fraction is demonstrated. A quantitative criterion for the occurrence of viscosity anomalies is given in terms of the reduced melting temperature. This criterion agrees with the model of restricted molecular rotation (Matheson and co-workers) in the prediction of viscosity anomalies, and it has also been found to hold in cases where classifications according to the entropy of fusion and the molecular sphericity fail.