Thanks, I understood the big/old small/young use. What I meant is how can you tell whether ...'büyük mü?' means 'is it big/old?' in the 3rd person or 'are you big/old?' in the 2nd person with the -sün left off after the first half of the question? E.g. if someone said 'I can walk into my wardrobe', you might reply 'Really? Are you small or is it big?' Or would you say that a different way?
This is a great question and leads to an important point in Turkish grammar. If you have to different subjects in one sentence in Turkish, you must state them. Otherwise, you will get confused like this and people will just understand one of them :)
So what you want would be "sen küçük müsün, (yoksa) o büyük mü?" :)
It comes after every choice you give in a multiple choice question. (Example: "Are you drinking coffee, tea, milk or orange juice?" = "Kahve mi, çay mı, süt mü yoksa portakal suyu mu içiyorsun?") It may seem awkward at first, but I think it actually helps you keep track when you start putting together more complex sentences with confusing word orders.