Word Distinction: Body Shape

Thin is a general word to describe people who have little or no fat on their bodies. If someone is thin in a pleasant way, we say they are slim (or less common) slender.
I wish I were as slim / slender as you.
We could also say lean (= thin in a strong and healthy way):
a lean, muscular body
If they are too thin, they are skinny (= informal), underweight or, worst of all, emaciated:
He looks very thin/skinny/emaciated after his illness.
The prisoners were emaciated.
Thin can be used for things:
a thin pole
but not usually for flat surfaces (especially surfaces where a person might go) or for openings. Instead we say narrow:
a narrow road / a narrow bed
Fine is used to describe that are thin when you are giving the idea of careful, sensitive work
She drew with a fine pen.
fine silk thread