Enhancing the filler-to-matrix contact area by incorporating large graphene flakes (GFs) is considered a viable approach for improving the mechanical properties of polymer composites. Graphene oxide (GO) flakes have been initially pursued for this application owing to the advantage of producing large-sized GFs. However, the defective nature of GO makes it technically challenging to precisely assess their dimensional effect on resulting mechanical properties, limiting their versatility as reinforcing materials. Therefore, it is highly desired to fabricate GFs with minimum defects and large lateral size. In this paper, we report the fabrication of high-quality edge functionalized non-oxidized graphene flakes (f-NOGFs) by the liquid exfoliation of graphite intercalation compounds. As-prepared f-NOGFs are over 1 μm in lateral size and contain less than 6.25% of impurity oxygen. By using size-sorted f-NOGFs, we systematically studied the relationship between mechanical properties and lateral sizes in f-NOGFs-incorporated epoxies. We identify that epoxy composites containing 0.6 wt% of f-NOGFs with a lateral size of 1 μm present outstanding mechanical properties; elastic modulus of 3.65 GPa, ultimate tensile strength of 95.74 GPa and toughness of 2.52 MJ m⁻³. The study presented in this paper could provide better understanding for optimization of the mechanical reinforcement of graphene-polymer composites.