In this paper, the influence of the aortic dimensions of an investigated mouse on its resulting wall shear stress (WSS) was studied. A numerical model of a mouse aortic arch was created based on a micro-CT scan of a vascular corrosion cast of an 8-week-old wild type mouse. This model was then rescaled to obtain five models with aortic root diameters corresponding to five different stages in the mouse life cycle varying from late fetal (0.7 mm) to old adult (1.5 mm). Consistent with literature, WSS values much higher than those normally encountered in humans were found. WSS was found to decrease rapidly in early life stages and to reach a plateau in adulthood, thus supporting a mediating role for WSS in arterial growth. Our results show that WSS values for mice should be interpreted very cautiously, and if possible an animal-specific geometry with animal-specific boundary conditions should be used.