The effect of catalyst particle size on thermodynamic equilibrium of methane dry reforming and carbon formation has been studied through the Gibbs free energy minimization method taking into account the deviation of carbon formed from graphite Gibbs energy and its dependence on catalyst particle size. Methane and CO2 conversions are maximized at low pressure and high temperature, and a molar H-2/CO ratio of 1 is obtained at 1100-1200 K and 5-10 bar. Carbon formation was found to increase with particle diameter, and carbon presence was noticed at conditions of high pressure/low temperature and high temperature/low pressure. Optimal operating conditions were found to be close to carbon limits, highlighting the need for active metal particle size to be less than 5-6 nm to minimize coking. CO was identified as the precursor for carbon at low temperature, while CH4 was found to be the main precursor at high temperature.