Aim: To evaluate the relationship between tumor size and survival in metastatic colorectal cancer (mCRC) patients who received chemotherapy. Materials & methods: SEER database was accessed for eligible patients. Multivariate Cox regression analysis was performed to compare the effect of tumor size on overall survival (OS) and CRC-specific survival (CCSS). Results: Tumor size ≥5 cm was an independent risk factor for OS and CCSS in mCRC patients treated with chemotherapy. Tumor size <5 cm did not show a survival advantage in patients whose primary tumor site was rectosigmoid junction, while tumor size ≥5 cm was associated with poor OS and CCSS in left-and right-sided colorectal cancer. Conclusion: Tumor size ≥5 cm was associated with poor prognosis after receiving chemotherapy treatment and a risk factor for survival of mCRC.