Dielectric measurements of PbMg1/3Nb2/3O3(PMN) powder and dense ceramics with grain sizesbetween 15nm and two microns were carried out in a broad frequency range (20Hz-1GHz). Clear grain size dependence of relaxor behavior was evidenced. A progressive transformation from Vogel-Fulcher behavior towards the Arrhenius process in the PMN with reduction of grain size in both ceramics and powder was observed. In the case of ceramics we were able to extract deeper information from the distributions of relaxation times and an analysis using the Vogel-Fulcher law, revealig two main contributions: a fast part of distribution of relaxation times with a maximum close to 10-11 s, wich is almost grain-size independent and has a non-polar origin;whereas, a process with long relaxation times (in the time range of 10-8 to 10-5s) si associated with the dynamics of the polar nanoregions and is strongly suppressed with reduction of grain size. The results of dielectric investigations are confirmed by Nuclear Magnetic Resonance experiments.