Formation of cellular detonation in a stoichiometric mixture of aluminum particles in oxygen is studied by means of numerical simulation of shock-wave initiation of detonation in a flat and rather wide channel. By varying the channel width, the characteristic size of the cells of regular uniform structures for particle fractions of 1-10 µm is determined. The calculated cell size is in agreement with the estimates obtained by methods of an acoustic analysis. A relation is established between the cell size and the length of the characteristic zones of the detonation-wave structure (ignition delay, combustion, velocity and thermal relaxation).