"Are you big?"
Nope, then you get no meaning of "you." What you wrote would be "Is he/she/it big?"
Vowel harmony :) The final vowel in "Büyük" is -ü, so this carries over into the question particle.
"Are you big?" Translation: Büyük müsün?
Please can I add to what miss has said.
Back vowels: (a, ı, o & u) back vowels can only be followed by a back vowel. Example, uyanınca: uyan – wake up sleepy head, uyanın – wake up sleepy heads & uyanınca – awakened sleepy head/heads. All the vowels in the word uyanınca are back vowels. More examples: Kalkınca, soruyor, kırılsam, olmadın, olanları, kulum, kullarını & tanrım. Just to confuse you or get you thinking? The question contains front vowels that can only be followed by front vowels.
So: Buyuk misin?? isn't correct?? why?? getting confused. - Wrong.
Büyük müsün? - Two front vowels in "büyük." "ü" x 2. The following word can only be "müsün" as front vowels can only be followed by front vowels.
The Turkish alphabet (Turkish: Türk alfabesi) is a Latin-script alphabet used for writing the Turkish language, consisting of 29 letters, seven of which (Ç, Ş, Ğ, I, İ, Ö & Ü) have been modified from their Latin originals for the phonetic requirements of the language.
misin - contains a front vowel "i" & your homework is to figure out why this word was incorrectly used?