The grain size of thermally crystallized a-Si films is controlled by the nucleation, rN, and growth, rG, rates according to the standard Avrami's theory. Despite this evidence, most papers devoted to improve the crystallized grain size analyze their results with a qualitative reference to this theory. In this paper, we will show that one can identify the standard set of rN and rG values for a-Si and that experiments show that deviations from this standard values always result in a smaller grain size. It is also shown that one cannot expect any substantial improvement with non-conventional heat treatments. Finally, it is argued that a larger grain size is expected from a-Si films containing, in their as-grown state, a controlled density of embedded nanocrystals.