As a part of serial work about the application of a tetra-needle-shaped whisker (T-ZnOw) with a spatial structure in polymer composites, this work is focused on the effect of T-ZnOw dimension on the properties of polymer composites. Two kinds of T-ZnOw whiskers with different dimensions (big and small one) are introduced into PA6 composites. Our results show that for the big whisker and the small whisker both, the addition of T-ZnOw induces the mechanical property improvement of T-ZnOw/PA6 composites. Especially, the reinforcement effect of small whisker is more apparent than the bigger one. The toughening mechanism of T-ZnOw is comparatively studied through the analyzing of impact-fractured surface morphologies and the single-edge-single-notched 3-point-bend test. Finally, the effect of T-ZnOw on thermal properties of the composite is also investigated, and the results show that small whisker reinforced composites exhibit much higher heat deflection temperatures compared with the big whisker reinforced composites possibly due to the greater volume effect.