Composites are essential in a wide range of engineering fields, but their mechanical behaviour is difficult to be characterised due to the influence of their microstructure on the macroscopic response of the material. Classical continuum model often proves inadequate to describe such materials, particularly when it is necessary to account for micro-level features, such as the orientation and size of heterogeneities. To overcome this limitation, the micropolar model has shown effectiveness in representing materials mechanical behaviour accounting for size effects. This study focuses on determining the micropolar elastic constants of composites represented by rigid blocks which interact through thin elastic interfaces. A heuristic optimisation technique, based on the Differential Evolution algorithm, is used to estimate the micropolar parameters by exploiting results from static and dynamic analyses conducted on discrete models, which offer a finer material description by distinguishing between blocks and interfaces.