Despite being commonly employed to treat peripheral artery disease, self-expanding Nitinol stents are still associated with relatively high incidence of failure in the mid- and long-term due to in-stent restenosis or fatigue fracture. The practice of stent oversizing is necessary to obtain suitable lumen gain and apposition to the vessel wall, though it is regarded as a potential cause of negative clinical outcomes when mis-sizing occurs. The objective of this study was to develop a computational model to provide a better understanding of the structural effects of stent sizing in a patient-specific scenario, considering oversizing ratio OS, defined as the stent nominal diameter to the average vessel diameter, between 1.0 and 1.8. It was found that OS < 1.2 resulted in problematic short-term outcomes, with poor lumen gain and significant strut malapposition. Oversizing ratios that were in the range 1.2 ≤ OS ≤ 1.4 provided the optimum biomechanical performance following implantation, with improved lumen gain, reduced incomplete stent apposition and favourable predicted long-term fatigue performance. Excessive oversizing, OS > 1.4, did not provide any further benefit in outcomes, showing limited increases in lumen gain and unfavourable long-term performance, with higher mean strain values predicted from the fatigue analysis. Therefore, our findings predict that the optimal oversizing ratio for self-expanding Nitinol stents is in the range of 1.2 ≤ OS ≤ 1.4, which is similar to clinical observations, with this study providing detailed insight into the biomechanical basis for this.