In biotechnology, deterministic lateral displacement (DLD) is used to separate label-free particles - such as cells, bacteria and exosomes - according to their size. The separation phenomenon is based on steric effects around an array of shaped micro-pillars: particles larger than a critical size are laterally displaced by the pillars whereas smaller particles follow a global straight path. Models have been proposed in the literature to predict the critical sizes according to the geometry of the DLD arrays. However, these models are semi-empirical and do not cover all the possible pillar shapes and orientations. Here we present a finite element model to find the best separation geometries for different-shaped micro-pillars, using COMSOL finite element particle tracing module. Calculation results are compared to the literature and confirmed by experimental results with calibrated polystyrene particles. Therefore, the presented model allows to choose the optimal DLD design according to the desired separation size.