But I wonder why the only description of a character’s looks was Remus thinking of her mother's size and hair-colour. Why would he do that, and why would we need that information now that we had already met her in the first installment of the story?

Well, I was going for "she gives the impression of being very prim and proper" without actually saying so in as many words, but maybe it didn't work properly. Hmm. (And I see I'm terrible at describing settings, as usual. Thanks for flagging that.)

Thank you for your (extensive) comments!