The smallest size of a spin crossover particle which still exhibits hysteric behavior is an open question. It is clear that spin crossover is a cooperative phenomenon, which implies the dependence of the high spin fraction variation on the number of molecules within the system. We use the "atom-phonon coupling" model to explain the different results that have been recently published with regard to the critical dimension of a spin crossover system. In the second part of the paper, the applicability of the model to the pressure induced transition has been developed, and the results are in good agreement with what has been observed experimentally. © 2008 American Institute of Physics.