Background: In staging for testicular germ cell tumor (GCT), current guidelines lack consensus regarding the measurement of retroperitoneal lymph node metastasis, concerning the recommended plane and dimension. This exploratory study aimed to assess its impact on clinical stage (cS) and therapy. Methods: We retrospectively examined 154 cSI (retroperitoneal lymph nodes < 10 mm in axial short-axis diameter (SAD)) GCT patients, without adjuvant therapy and a follow-up ≥ 24 months. Retroperitoneal lymph nodes were measured in staging images in different dimensions (SAD and long-axis diameter (LAD)) and planes (axial, sagittal and coronal). Results: Overall survival was 100%, with 82% free of recurrence after a median follow-up of 83 months. All patients were classified as cSI, based on axial SAD (RECIST 1.1). However, significantly more patients would have been classified as cSIIA (0% vs. 38% vs. 52%) or even cSIIB (0% vs. 1% vs. 25%) according to axial LAD (SWENOTECA, German S3 guideline) or maximum LAD in any plane (EAU, ESMO, AJCC and onkopedia) (p < 0.001). Overtreatment was predicted in 0%, 31% and 61% of patients based on axial SAD, axial LAD and maximum LAD, while undertreatment was estimated at 18%, 10% and 2%, respectively, (p < 0.001). Conclusions: These findings indicate considerable variability in cS based on current lymph node staging recommendations, suggesting that axial SAD (RECIST 1.1) could be the most appropriate parameter for standardized guideline recommendations.