We employed electron spin resonance (ESR) technique in the investigation of a series of graphene flakes with different size distributions. Graphene oxide (GO) samples with three distinct size distributions were prepared by the Brodie method. Large flakes of similar to 2500 mu m(2), intermediate size flakes of similar to 1 mu m(2), and small ones of 500 nm(2) have been characterized by ESR in the 4-295 K temperature range. Already for GO flakes the magnetic response strongly varies with their size. Large GO flakes are inhomogeneous, revealed by the presence of both Pauli and Curie components in the overall signal. In small GO flakes ESR spin susceptibility reveals superparamagnetic behavior ascribed to interaction between localized spins. The intermediate flakes show a standard Curie behavior showing a uniform oxidation and the concomitant non-interacting localized spins. On this batch a thermal method of reduction was applied such that in polar solvent the suspension of GO was heat treated at temperatures below 200 degrees C. ESR susceptibility shows that this reduction can recover some of the Pauli-like contribution of the pristine graphene. However, our study shows that the full optimization of oxidation/reduction parameters is yet to be achieved. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim