"The women's big skirts"
Translation:Les grandes jupes des femmes
Well, you are right. In another exercise there is also "big skirt" phrase and "grand" is accepted for French translation. DL consistency ;) ;)
Actually it seems that Duo is limited in what it can correct so it picks one thing. Since the grandes must come before the noun, it is wrong. Longues can come after the noun and would be correct.
I read that "gros" is about volume and "grand" about height. Can you explain it please?
Yes, "gros" is about volume: une grosse pomme, une grosse somme d'argent, un homme gros, un gros cheval
"grand" is about height or length or thinness, or greatness: une grande tour (high), une grande rivière (large), un grand artiste (great)