A nonlinear model for electrostatically actuated beams made of nanocrystalline materials is proposed. Two sets of measures are incorporated in the context of the proposed model. All the measures presented in the micromechanical model are related to the material structure. A size-dependent Euler-Bernoulli beam model is developed to model the mechanics of the nano-beam. The impacts of the material structure and size on the nonlinear dynamics and pull-in instability are studied.