Graphene has the potential to act as a high-performance reinforcement for adhesives or fibre composites when combined with epoxy polymer. However, it is currently mostly available not as single high aspect ratio sheets but as graphene nanoplatelets (GNPs), comprising stacks of graphene sheets. GNPs of a range of lateral size, thickness, aspect ratio and surface functionality were used to modify an anhydride-cured epoxy polymer. The morphology, mechanical properties and toughening mechanisms of these modified epoxies were investigated. The GNPs were sonicated in tetrahydrofuran (THF) or n-methyl-pyrrolidone (NMP) to facilitate dispersion in the epoxy. The use of THF resulted in large agglomerates, whereas more finely dispersed stacks of GNPs were observed for NMP. The maximum values of modulus (3.6 GPa at 1 wt%) and fracture energy (343 J/m2 at 2 wt%) were measured for the epoxy modified with an intermediate platelet size of approximately 4 μm, compared to 2.9 GPa and 96 J/m2, respectively, for the unmodified epoxy. The Young’s modulus was highly dependent on the dispersion quality, whereas the fracture energy was independent of the degree of GNP dispersion. The larger agglomerates of the GNPs which were dispersed in THF toughened the epoxy by crack deflection, whereas the GNPs dispersed in NMP showed platelet debonding, pull-out and plastic void growth of the epoxy. This work indicates that reinforcement and toughening can be achieved at much lower contents than for conventional modifiers. Further, achieving a good dispersion is crucial to the engineering application of these materials, and intermediate-sized graphene achieves the best balance of properties.