The effect of the lateral dimensions of the graphene reinforcement on the physical properties of poly(methyl methacrylate) (PMMA) nanocomposites was studied. Electrochemically-exfoliated, few-layer graphene with average diameters of 5 mu m (5-FLG) and 20 mu m (20-FLG) were incorporated into a PMMA matrix at loadings from 0.5 to 20 wt.% by melt mixing using a twin screw compounder. The smaller graphene flakes were found to have only a slight effect on the rheological properties of the PMMA, whereas the larger flakes significantly increased the viscosity and dynamic moduli of the PMMA melt. Mechanical testing showed that the 5 mu m flakes behaved in a similar way to short fibers and reinforced the PMMA matrix poorly compared to the 20 mu m flakes, in line with previous predictions on the critical length of graphene. Significantly, these larger flakes gave a linear increase in the modulus with loading even at high loadings, without any of the detrimental aggregation effects seen in most other graphene systems. (C) 2015 Published by Elsevier Ltd.