This study investigates the effect of air damping on in-plane silicon micro/nano-resonators sandwiched between two electrodes (two ports) for sensing and actuation. Experimental measurements are presented for the quality factor (Q) as varying pressure for several case studies of clamped-clamped and clamped-free micro/nano-beam resonators of various geometrical parameters and airgap dimensions. The focus of this work is on large airgap dimensions, where typically squeeze-film damping is assumed negligible. In addition to the fundamental first mode, several results are shown when the resonators are operated near their second or third modes of vibrations. Several curves are generated to show the dependence of the quality factor on the resonator size, boundary condition, and mode order. Several analytical models are applied to investigate the dominant dissipations mechanisms and the models capability to predict Q on both low and higher pressure regimes, and the results are compared to the experimental data.