Donor to recipient undersizing can result in diminished graft survival. The United Network for Organ Sharing database was retrospectively queried from January 2008 to December 2013 to identify adult patients who underwent heart transplantation. This population was divided into those without and with a left ventricular assist device (LVAD) at the time of transplant. Both groups were further subdivided into three groups: donor:recipient body mass index (BMI) ratio <0.8 (undersized), ≥0.8 and ≤1.2 (matched), and >1.2 (oversized). Kaplan–Meier analysis was used to compare graft survival. Cox regression analysis was used to identify factors affecting graft survival time. There was no difference in mean graft survival between undersized, matched, and oversized groups in patients without an LVAD (p = 0.634). Mean graft survival was significantly worse for undersized patients with an LVAD when compared with matched and oversized patients (p = 0.032). Cox regression revealed age, creatinine, waitlist time, United Network for Organ Sharing status, BMI ratio, and total bilirubin as significant factors affecting graft survival time. A donor to recipient BMI ratio of ≥1.2 results in significantly improved long-term graft survival for patients with an LVAD at the time of heart transplantation compared with patients with a BMI ratio of <1.2. An oversized organ should be considered for patients supported with an LVAD.