The rheogram, shear stress versus shear rate of magnetic suspensions composed either of monodisperse or polydisperse spherical particles, has been measured for different particle sizes. For small particles, where the ratio λ of magnetostatic energy to thermal energy is already much larger than unity (λ≊102–103), we find that the apparent yield stress is still strongly increasing with the size of the particles. It is only for very high values of λ (λ≊109) that we find experimental results which show no dependence on the size of the particles. These results are discussed with the help of a structural model based on the deformation of simple chains of particles, with an interparticle distance which depends on λ. It is shown that the experimental results can be explained by taking into account the fluctuations of the positions of the particles in a chain.