Our book passage #'s may be different because I don't have a passage XI but I'm going to assume you're talking about the recrystallization passage (correct me if I'm wrong).
For that passage the last paragraph does clump melting point + crystal size info together but they're actually two different concepts. So yes, a smaller mp range corresponds to a more pure crystal but the longer crystals tidbit relates to recrystallization not melting (you're melting them anyway, can't see if they're long or cubic if they're a liquid puddle). A small mp range indicates purity since compounds only have one mp.
In recrystallization longer crystals indicate a more pure compound because it means more of the like molecules have come together in a crystal lattice. If you have many impurities it would disrupt this lattice and you'd get smaller bits. This process has to do with cooling the compound rather than melting it. Does that make any sense?
