Cast aluminium components may exhibit material imperfections such as shrinkage and gas pores, or oxide inclusions. Therefore, the fatigue resistance is significantly influenced by the size and location of these inhomogenities. In this work, two different specimen geometries are manufactured from varying positions of an Al-Si-Cu alloy casting. The specimen geometries are designed by means of shape optimization based on a finite element analysis and exhibit different highly-stressed volumes. The numerically optimized specimen curvature enforces a notch factor of only two percent. To enable the evaluation of a statistical size effect, the length of the constant testing region and hence, the size of the highly-stressed volume varies by a ratio of one to ten between the two specimen geometries. Furthermore, the location of the crack initiation is dominated by the comparably greatest defects in this highly-stressed volume, which is also known as Weibull's weakest link model. The crack initiating defect sizes are evaluated by means of light microscopy and modern scanning electron microscope methods. Finally, the statistical size effect is analysed based on the extreme value distribution of the occurring defects, whereby the size and location of the pores is non-destructively obtained by computed tomography (CT) scanning. This elaborated procedure facilitates a size-effect based methodology to study the defect distribution and the associated local fatigue life of CPS casted Al-Si lightweight components.