Accurate reporting of the residual tumor size by pathologists after neoadjuvant chemotherapy is an important component of a breast cancer. Recent literature reported comparisons regarding the accuracy of clinical and radiological residual tumor size findings using the histopathology as a "gold standard". However, the histopathological methods of measuring the residual tumor size are not standardized. Most pathologists use the tumor size measured by the gross examination. We collected 32 patient samples and compared the residual tumor size by gross and microscopic pathologic examinations. Using microscopic tumor size as the gold standard, our study showed gross tumor size is overestimated in 25%, underestimated in 56% and correlated to the final microscopic tumor size in 19% of the cases after neoadjuvant chemotherapy. Determining accurate residual tumor size to estimate pathologic response to chemotherapy is essential. We attempted to provide guidelines for pathology reporting post-neoadjuvant chemotherapy on breast cancers.