Introduction: The presence of extranodal extension (ENE) in sentinel lymph nodes (SLNs) can predict non-SLN metastases in breast cancer (BC) patients; however, the prognostic relevance of its extent remains controversial. The purpose of this study was to examine the predictive role of ENE in SLNs measured by its widest dimension (WD), highest dimension (HD), and the WD/HD ratio for non-SLN involvement, overall, and disease-free survival (OS, DFS) in cT1-2N0 BC patients with positive SLNs. Materials and methods: A total of 511 women with cT1-2N0 BC and positive SLNs undergoing axillary lymph node dissection were retrospectively enrolled. The associations of ENE's WD, HD, and WD/HD ratio with non-SLN metastases, 5-year OS, and DFS were established through a multivariable modeling approach. Results: SLNs were ENE-positive in 149 (29.16%) participants, and 133 (26.03%) had non-SLN metastases. During the median 60 (16-60)-month follow-up, 69 (13.50%) patients experienced recurrences, and 62 (12.13%) died. The numbers of SLNs, non-SLNs, and total axillary LNs involved differed between the ENE-negative and ENE-positive groups, as well as between the WD/HD ≤ 1.2 and WD/HD > 1.2 subgroups (all P-values were < .001). Multivariable analyses showed significant associations of the WD/HD ratio > 1.2 with non-SLN involvement, OS, and DFS (P-values were .003, < .001, and .005, respectively). Discussion: Despite no predictive value of ENE's WD and HD, the WD/HD ratio > 1.2 was an independent predictor of non-SLN involvement, mortality, and recurrence. ENE's WD/HD ratio could be a valuable indicator for cT1-2N0 BC individuals with positive SLNs for whom further axillary treatment may be beneficial.