The investigation of detonation in hydrogen-nitrous oxide mixtures is related to several important industrial safety issues including nuclear waste storage and semi-conductor production. Because of a limited number of detonation cell size data available in the literature, the present study aims at providing additional experimen- tal data and estimating several methods to predict detonation cell size. Detonation cell size is measured at different argon dilution and initial pressures, for mixtures with equivalence ratios from 0.3 to 2.5. It is shown that even at low initial pressure, H2-N2O-Ar mixtures are very sensitive to detonation. Using a detailed chemical kinetic model, the semi-empirical correlation by Ng has been shown to give accurate prediction of cell size. After a specific reduction of the detailed scheme and the adjustment of the detonation velocity to match the experimental one, a 2-D Euler simulation has been shown a capability to provide reliable cell size data for stoichiometric mixtures.