The size distribution of methanogenic granules from ten full scale UASB and Internal Circulation (IC) reactors were studied at regular time intervals. A size distribution model was developed based on these data and additional lab-scale experiments. Full scale data were used, since the hydrodynamic conditions in full scale reactors influencing the size distribution cannot be simulated properly in lab scale reactors. It was concluded that breaking and/or disintegration did not significantly contribute to the size distribution. Washout of granules is under normal conditions negligible, since the settling velocity is much higher than the liquid upflow velocity. Gradient measurements showed that the sludge bed is well mixed. Thus, sluicing will not systematically remove large granules. The most significant process limiting the maximum granule size in normal operation is the regular sluicing of surplus biomass. Shear forces appear to have no influence on the size distribution. High numbers of precursors in the influent result in short size distributions, while in wastewaters with little or no suspended solids wide size distributions can be found. The developed size distribution model accurately described the wider size distributions found in IC-reactors. IC-granules are larger and less strong as compared to UASB granules. Furthermore, it was concluded that operating the systems at high loadings results in granules with a lower strength. The density of methanogenic granules is strongly correlated with the ash content.