This study aimed to evaluate the impact of the diverse stent size selection on the clinical and angiographic outcomes of Willis covered stent (WCS) for the treatment of skull base cerebrovascular diseases. A total of 147 patients with 151 skull base cerebrovascular diseases treated with WCS in 3 centers between January 2015 and July 2022 were included in this study. Several parameters depicting stent size and parent artery condition were incorporated into the analysis of the outcomes. Complete occlusion was found in 106 cases (68.2%) immediately after deployment and 126 cases (83.4%) after technical adjustment. In the multivariate logistics analysis, the difference between stent diameter and parent artery diameter (DD) was significantly associated with immediate endoleak without adjustment (odds ratio [OR]=0.410; p=0.005) and late endoleak (OR=0.275; p=0.028). In addition, differences between stent diameter and parent artery diameter at wide landing point (DSW) and differences between stent diameter and parent artery diameter at narrow landing point (DSN) was also was significant associated with immediate endoleak without adjustment and balloon re-dilation respectively. This study demonstrated that the diameter selection of the WCS was associated with the occurrence of endoleak during the treatment of skull base cerebrovascular diseases. Precise selection and evaluation of stent size and vessel condition were significant factors for skull base cerebrovascular diseases treated by WCS. This study demonstrates a significant association between the diameter selection of the Willis covered stent (WCS) and the occurrence of endoleak in the management of skull base cerebrovascular diseases. The results offer valuable medical evidence that can inform stent selection for WCS. The study emphasizes the significance of precise evaluation of stent size and vessel condition as crucial factors in WCS procedures. These findings underscore the importance of meticulous consideration and individualized approaches to stent selection, ultimately improving treatment outcomes in clinical practice.