The present study investigates the nonlinear size-dependent behaviour of an electrically actuated MEMS resonator based on the modified couple stress theory; the microbeam is excited by an AC voltage which is superimposed on a DC voltage. A high-dimensional reduced order model of the continuous system is obtained by applying the Galerkin scheme to the nonlinear partial differential equation of motion. The pseudo-arclength continuation technique is employed to examine the nonlinear static and dynamic behaviour of the system. Specifically, the nonlinear static behaviour of the system is investigated when the microbeam is excited by the electrostatic excitation (DC voltage); this analysis yields the static deflected configuration of the system and the value of the DC voltage corresponding to the static pull-in instability. The size-dependent dynamic behaviour of the system is examined under primary and superharmonic excitations; the frequency- and force-response curves of the system as well as time histories and phase-plane portraits are constructed. Moreover, the effect of taking into account the length-scale parameter on the static and dynamic behaviour of the system is examined by comparing the results obtained by means of the classical and modified couple stress theories.