Accurate estimation of tumor size is a challenging task due to the large variation in shape of prostatic tumors. Measurement of diameter and volume of tumor are useful for the staging of prostate cancer(PCa). Therefore, an appropriate method for tumor assessment is essential to assist clinical management of PCa. The goal of this study was to investigate the role of tumor size estimation methods (2D vs. 3D) for the Prostate Imaging Reporting and Data System version-2(PI-RADS v2) assessment of PCa using multiparametric MRI. Automatic tumor size assessment using ellipse-fitting approach showed relatively better performance of PCa assessment compared to radiologist-assessment.