To investigate the impact of transcatheter heart valve (THV) sizing on procedural results and clinical outcomes following transcatheter aortic valve implantation (TAVI). The impact of individual THV sizing for patients with borderline aortic annulus anatomy remains unclear. In the prospective BernTAVI registry, THV sizing conditions were retrospectively evaluated, and patients were categorized into three groups based on the recommendations and the sizing chart of the manufacturers: optimal sizing, borderline sizing (THV size located within 5% to each border of the optimal sizing recommendation), and suboptimal sizing (THV size outside the recommended range). The latter two groups were further subcategorized into THV-oversizing and THV-undersizing. The primary endpoint was a composite of all-cause death and unplanned repeat intervention at 1year. Out of a total of 1,638 patients who underwent TAVI, 9.5 and 15.6% of patients were categorized into the borderline and suboptimal sizing group, respectively. Device success was achieved in 87.4, 88.9, and 83.6% of patients with optimal, borderline, and suboptimal sizing, respectively. The primary endpoint occurred in 12.3% of patients with optimal sizing, 14.9% of patients with borderline sizing (HRadj 1.35, 95%CI 0.87-2.09), and in 17.4% of patients with suboptimal sizing (HRadj 1.42, 95%CI 1.01-1.99). Within the suboptimal sizing cohort, unfavorable outcomes were mainly associated with THV undersizing (device success: 76.4%, primary endpoint: 23.9%, HRadj 1.98, 95%CI 1.36-2.87). Suboptimal TAVI prosthesis sizing is associated with an increased risk of all-cause death and unplanned repeat intervention within 1year largely attributable to undersized THV prostheses.