You might be referring to this discussion: https://forum.duolingo.com/comment/401417/She-has-blue-eyes
I think the most relevant part is this:
For body parts, we use definite articles with verb "avoir". il a la mâchoire carrée, les yeux bleus, les cheveux blonds, les épaules larges...
Another useful one here: https://forum.duolingo.com/comment/210450/Elle-a-de-grands-yeux-bleus
"elle a les grands yeux bleus" would translate "she has the big blue eyes" which would miss something to specify the use of the definite article:
elle a les grands yeux bleus de sa grand-mère
In this sentence, "de" is used instead of "des" because the adjective is in front of the noun, and it is simply the plural of "un": "elle a un grand nez" (she has a big nose).
and further down:
I have blue eyes / my eyes are blue = j'ai les yeux bleus / mes yeux sont bleus
I have big eyes / my eyes are big = j'ai de grands yeux (Size adjective, placed in front of the noun) = mes yeux sont grands
I have big blue eyes / I have big blue eyes = j'ai de grands yeux bleus (ditto) = mes yeux bleus sont grands
It looks like it depends on whether an adjective that goes before the noun appears or not.