The effect of aggregate size on ASR expansions has been largely studied and conflicting results exist concerning the aggregate size that leads to the highest ASR expansion. Most of the research works clearly show a pessimum effect of aggregate size on ASR expansion. However, all the results available in the literature were obtained using different experimental conditions and the combined effects of other important parameters, such as specimen size used in the expansion tests, have often been neglected. This paper aims to investigate the combined effect of specimen size and aggregate size on ASR expansion. Experimental results highlight a scale effect, a combination of the effects of aggregate size and specimen size on ASR expansion. This scale effect appears to be influenced by the reactive silica content of the aggregate. Modelling at microscopic level is used to propose a quantification of this effect.