Background: Pathologic staging in colorectal cancer (CRC) is crucial in patient management. Data regarding the impact of size/horizontal tumor extent is limited, contradictory and currently excluded from the American Joint Committee on Cancer (AJCC) staging model. However, a previously published SEER analysis showed that AJCC stages I and IIIA have similar 2- and 5- year survival rates, and worse rates for stage II. Using the largest cohort to date, we report the impact of primary tumor size on CRC survival. Methods: Data were obtained from all US hospitals that contributed to the National Cancer Database (NCDB) between 2010 and 2015. Univariate and multivariate analyses were performed to identify factors associated with patient outcome. Kaplan-Meier analysis and Cox proportional hazards models were used to assess the association between tumor/patient characteristics and overall survival (OS). Results: A total of 61,145 patients were identified with a similar gender distribution (M/F:50.9%/49.1%). The mean age was 62.7years (SD+/-14.1) and 82% were non-Hispanic Whites. Majority had colon primary (82.7%) and 82.4% had microsatellite stable (MSS) disease. Distribution across stages I-IV was 20.1%, 32.1%, 34.7% and 13.2% respectively. Among the total study population, AJCC stage correlated closely with OS on multivariate analysis (HR 1.49, 2.29, 8.38 for stages II to IV compared to stage I), while the distinguishing power for tumor size was relatively mild (HR 1.19 and 1.33 for 5-10 cm and >5cm compared to <5cm). Among patients with stage II disease, tumors >10cm were associated with worse survival compared to those <5cm (HR 1.2; 1.03-1.39; p=0.22). Stage III disease also had differential survival rates; patients with tumors 5-10cm (HR 1.21; 1.14-1.28; p<0.001) and >10cm (HR 1.57; 1.37-1.80; p<0.001) had worse survival than those <5cm. Patients with stage II who did not receive adjuvant chemotherapy (CTX) had worse survival outcomes (HR 1.29; 1.08-1.55; p=0.005) compared to stage III disease who did. Accounting for tumor size, there was no statistically significant survival differences between stage I patients and stages II and III patients who received adjuvant chemotherapy. Conclusions: Tumors larger than 10cm have inferior outcomes among patients in the same AJCC stages. Stage II patients without adjuvant CTX did worse than stage III with CTX. Further studies are needed to clarify the role of tumor size in staging models.