The geometric proportions of cube-shaped specimens subjected to wedge-splitting tests are numerically studied in the paper. The minimal notch length for specimens made of cement based composites varying in characteristic length of the material (a measure of material brittle-ness/heterogeneity) is verified using finite element method code with an implemented cohesive crack model (ATENA). The problem of assigning the crack initiation point (the notch tip vs. the groove corner in the load-imposing area of the specimen) is solved numerically also using both the theory of linear elastic fracture mechanics and the theory of the fracture mechanics of generalized singular stress concentrators in the second part of the two-part paper. Results ob-tained by the different approaches are compared. The minimal notch length is recommended.