"Which ball is big?"
Translation:어느 공이 큽니까?
Okay, I'm quite sure you did not intend it, but it is actually correct. -ㅂ디까 is a 하오체 ending which our course does not cover. It is used when you are asking about the listener's personal experience. Since there are hundreds of different ways of asking questions in different situations, we cannot add all accepted translations, but it is likely that the algorithm considered 큽디까 as a typo of 큽니까.