First, let us review what an adjective is. Adjectives are words you use to describe nouns (or pronouns). If I write, "I just finished walking my noisy dog," the word "noisy" is an adjective telling you something about my dog (the noun).
In "Barbie Doll," Piercy uses adjectives to describe the girl she's writing about directly. She also uses adjectives to describe specific parts of the girl. Finally, she uses adjectives to describe objects belonging to the girl, which still end up describing her through implication.
Piercy describes the girl directly in lines like "She was healthy, tested intelligent." In that line, "healthy" and "intelligent" are adjectives describing the girl.
Piercy describes the girl's body parts in lines like "You have a great big nose and fat legs." There, "great" and "fat" are adjectives.
Piercy also describes the girl indirectly when talking about her possessions, with lines like "miniature GE stoves and irons/ and wee lipsticks the color of cherry candy." In that excerpt, "miniature" and "wee" both describe the girl's possessions, thereby describing her as small or diminished.
Now, if you want to come up with your own adjectives about the girl, you would read the poem and synthesize the points Piercy makes. The girl would be, for example, apologetic in the second stanza.