We evaluate Weibull’s approach and the coupled criterion (CC) ability to reproduce bending failure stress variations as a function of specimen size in two porous materials, namely gypsum or zinc oxide. Whereas both approaches well reproduce the size effect in gypsum specimens, only the CC succeeds in correctly predicting the failure stress variation for ZnO specimens. We thus question the basis assumption of increasing critical flaw size with increasing specimen size associated to Weibull’s approach. Employing the CC to predict prematurate specimen failure initiating from a critical pore, we determine a relation between the failure force decrease and the corresponding possible critical pore size range. For the three tested gypsum types, we estimate the critical pore size range to lie around 50–250 microns, not retrieving increasing critical pore size with increasing specimen size, as assumed in Weibull’s approach, but rather a constant critical pore size range for all specimens.