Purpose Tumor size is a key prognostic variable for soft tissue sarcomas (STS), and a tumor diameter of 5 cm is generally used as a cutoff for risk grouping purposes. This study aimed to investigate the prognostic effect of tumor size on overall survival and whether such an effect was influenced by the patient's size, expressed as body-surface area (BSA), in a series of 553 pediatric patients with localized STS. Patients and Methods Multivariable Cox models were used in which the effect of tumor size was adjusted for patients age, tumor site, histologic subtype, and Intergroup Rhabdomyosarcoma Study grouping, and the interaction between size and BSA was included. Results Tumor size was confirmed as a significant prognostic factor. Interaction between size and BSA meant that, for a given tumor size, the mortality increased from the larger to the smaller BSA. Conversely, the mortality risk associated with a patient with a BSA of 1.75 m² and a 5-cm tumor was the same as for a 0.6 m² child with a 2.8-cm tumor. Conclusion Tumor and body size are jointly informative on outcome. The risk associated with a given tumor size is not the same in patients of different body size, so it may be wrong to use the same 5-cm cutoff for tumor size in risk stratification.