We present the development of an original simulator to predict interferometric out-of-focus patterns created by irregular rough particles. Despite important simplifications of the scattering properties, this simulator allows to predict quantitative properties of the speckle-like patterns: i.e. the dimension of the central peak of the 2D-autocorrelation of the pattern. This parameter can then be linked to the size and the shape of the particle projected on the CCD sensor, in cases where there is no exact theoretical formulation to calculate the scattered intensity. An experimental demonstration is performed with irregular NaCl salt crystals.