Slim and slender.
narrow
Narrow
Narrow is used in the contexts of narrow roads or streets or a narrow-minded person. I guess narrow is used more figuratively and thin is used in physical contexts (thin body type, thin hair, thin atmosphere)
narrow valley
Spaghetti is thin and narrow; apply the sauce wide and thick.
A thin narrow strip of wood is either a lath or a slat.
Narrow, thin leaves have lower pressure under water.
skinny
thin
The word "thin" has numerous opposites: wide, thick, broad, fat, stout, hefty, and substantial; and the verbs thicken, widen, broaden, repopulate, replenish, compress, or concentrate.Conspicuously absent are antonyms that begin with the letter N. The closest might be natural for unthinned growth. Or the trivial "not thin."It is possible that this is asking for the synonym, not the antonym.(A close synonym is narrow.)
The opposite of narrow(thin) is wide.
A usable synonym would be small, and an antonym would be wide.