These saws are easy to use for pruning fruit trees.  The sizes S, M and L indicate the TPI (teeth per inch) for each saw.  The small size would have smaller space between each tooth, and the large would have bigger gaps.  The small size would make smoother cuts, whereas the larger size would offer a quicker cut, but rough cuts compared to the small and medium.  

Size	Pitch	TPI
Small (S)	12-14 Teeth/30mm	11
Medium (M)	11-12 Teeth/30mm	10
Large (L)	10.5-12 Teeth/30mm	9.5