Experimental and theoretical studies were conducted to simultaneously study the role of particle size and gas pressure on nonlinear behavior of dust oscillations in a plasma. Oscillation spectra were experimentally obtained by using four different sized (1.5, 2, 3, and 4 mum) particles at 250 mTorr, which is high pressure compared to previously reported works. The measured results were in good agreement with theoretical calculations based on a self-consistent collisional plasma model and a parametric dust oscillation model. In addition, particle size and gas pressure dependence of force profile and oscillation spectrum was investigated by numerical calculation in order to understand the role of particle size and pressure separately. It is concluded that occurrence of the subharmonic resonance and the net trapping force profile are mainly determined by particle size while gas pressure is mostly responsible for the superharmonic resonance and hysteresis. (C) 2004 American Institute of Physics.