Palladium clusters epitaxially grown on MgO(100) have been used as model catalysts to study the oxidation of CO by NO. The size distribution and the shape of the Pd panicles are determined by transmission electron microscopy (TEM). Three samples corresponding to particle mean sizes of 3, 7 and 16 nm have been studied. The adsorption of ND and its reaction with CO have been studied, at low pressure, using a molecular beam of NO and an isotropic pressure of CO. NO dissociates on all the particles between 170 degrees C and 450 degrees C. During the reaction, CO2 and N-2 are produced with a little amount of N2O below 300 degrees C. The curve of the steady state reaction rate as a function of the temperature has a "volcano" shape. The maximum of activity is shifted toward higher temperature when the CO pressure increases. The turnover rare for CO2 production increases when particle size decreases. However, as the reaction is limited at high temperature by NO and CO adsorption, it is necessary to correct the reaction rate by the fact that a large part of the reactant molecules adsorbed on the Pd clusters comes from the capture of molecules physisorbed on the support. After this correction, which depends on panicle size, the medium-sized particles appear to be the more active. A possible explanation based on the particle shape is discussed.