A recent model for the random walk of low energy electrons is applied to the crystallite-size effect on the secondary electron yield delta from insulators. The corresponding fitting procedure is successfully applied to the experimental results previously obtained by Ushio et al. on various MgO films (Thin Solid Films 167 (1988) 299). The values of the transport mean free path and of the maximum path length r for the SE in this type of MgO specimen are deduced for the room temperature. The difference between crystalline size effects and the influence of other parameters such as topographic effects or surface composition is established. Applications of the same model to X-ray induced electron emission yield delta(X) from insulators are also illustrated for MgO. Some other practical consequences, such as a new type of (crystallite size) contrast in scanning electron microscopy, are suggested. (C) 2003 Elsevier Science B.V. All rights reserved.