Crystallization using a co-solvent for producing spherical 3-nitro-1,2,4-triazol-5-one (NTO) crystals was carried out in two laboratory-scale and bench-scale crystallizers. The effect of scaling-up the crystallizers on the mean size of crystals of NTO during a batch cooling crystallization was explored. A simplified model based on the crystallization kinetics was established for the estimation of mean crystal size during the scaling-up of the crystallizer. Comparison of the crystals obtained from the laboratory-scale and from the bench-scale operations showed that the model can be successfully correlated with the mean crystal size. Eventually, the mean crystal size was estimated if the co-solvent composition was set optimally and the crystallizer scale-up was reasonable.