Translation:Are you big?
So from the comments this is asking if someone is old? How would you ask if someone is big, as in their size?
Is this a word one would use for people?
That's right. I forgot about that. I must admit that, if I heard this phrase, I would think fat, tall, or muscular, but not aged. How charming to refer to a shrunken old grandmother as a big lady.
So it certainly doesn't mean "Are you pregnant?". (Which was my first thought.)
It's interesting that "Büyük müsün?" wasn't translated as "Are you old?"... since "Küçük müsün?"" was translated as "Are you young?"!
Is "buyuk misin" correct? Exeuse me , Can you just clearly tell me what is the difference between misin and musun or misiniz and musunuz ?
"Büyük misin" isn't correct because we need to apply the 4-way vowel harmony. In this case the last vowel of the word before the question marker is "ü", so the right spelling is "müsün". Misin and musun mean exactly the same thing, it's just that we need to pick the right one based on the vowel harmony. Check the final vowel of the word before and apply the harmony based off that - e or i: use "misin"; o or u: "musun"; a or ı: "mısın"; ö or ü: "müsün"
The vowel has to agree with the vowel in the main body of the verb. There is no difference of meaning, just a difference of sound.