"she has a small bread" was fine… What? How could that possibly be the correct answer in English? I understand the "bread roll" translation, but letting "she has a small bread" be the correct answer is pretty grammatically incorrect in English, it definitely shouldn't be a "correct answer" option..
It depends on the masculinity of the item you're describing. If you say "Elle a du petit pain" which means "She has a small bread" or "She has a bread roll", you're putting the "petit" before the "pain" because you're talking about the small bread, right? You are not describing her, you're describing the bread. The bread is masculine which is why we use petit.