Background <p>Direct-to-implant (DTI) alloplastic breast reconstruction following mastectomy is a common surgical procedure. Implant selection is a key step in achieving a successful reconstructive outcome. Currently, there is no widely accepted system for implant size selection in reconstructive breast surgery.&#xa0;The aim of this study was to develop an implant size selection system for DTI breast reconstruction in patients with small and medium-sized breasts, based on anterior breast circumference measurements.</p> Methods <p>This was a two-phase observational study. Phase 1: Retrospective review of patients who underwent primary DTI breast reconstruction between January 2022 and June 2023.&#xa0;Phase 2: Prospective observational analysis of patients undergoing primary DTI breast reconstruction between August 2023 and August 2024.</p> Results <p>In the retrospective phase, 20 consecutive patients underwent DTI reconstructionusing the proposed implant size selection technique, achieving a 90% success rate. In the prospective phase, 22 consecutive patients were analyzed, with a 91% success rate. A successful measurement was defined as a case in which the preoperatively estimated implant size matched the implant used intraoperatively.</p> Conclusions <p>The technique described provides a structured implant size selection system for DTI breast reconstruction following mastectomy in small and medium-sized breasts. While most surgeons rely primarily on breast width and height, this method also incorporates the anterior skin envelope, a crucial factor in predicting postoperative breast volume. This system enables more accurate preoperative prediction of implant size, offering surgeons greater confidence in the expected reconstructive outcome.</p> Level of evidence <p>Level III, risk/prognostic study.</p>