The particle size and shape are of momentous significance for the mechanical properties of plastic composites. However, natural fillers, like wood, are not consistent in these attributes. In order to investigate the shared traits between these characteristics, WPCs were produced using polypropylene, hardwood and softwood fillers with different particle sizes and a coupling agent. Afterwards, specimens were processed using an injection molding machine. The filler sizes and shapes were measured using dynamic image analysis. Furthermore, a shortening of coarser particles was detected. Mechanical tests were carried out to gain information about the tensile, flexural and Charpy impact properties. Neither very coarse nor very fine particles showed the best results. Instead, medium-sized particles proved to be the best option. The evaluation of the particle geometry verified a definite correlation between the shape and the mechanical properties, especially regarding the convexity, which can be a useful indicator of the quality of fiber-matrix interaction. The fiber orientation in the matrix was visualized with images taken by X-ray micro tomography.