Segregated and dispersed oil-water flow through horizontal pipes is persistent. However, the prediction of operational parameters such as water volume fractions could be a difficult task. Therefore, in this study, an optimization model based on Computational Fluid Dynamics (CFD) is proposed in order to determine the droplet size distribution for modeling dispersed flows, which may help to accurately predict water volume fractions, especially when experimental data do not report the droplet size. The findings suggest that for the modeling of segregated flows, inlet droplet diameters near to 10 mm should be considered, while for fully dispersed flows, droplets diameters around 1 mm should be used. CFD validation shows a fair agreement with reported experimental data of water volume fraction, with errors below 14%.