Can we say: "Are you?" as a translation???
No, siz misiniz should be translated to "Is it you?"
Then how would you translate "are you"? Like in "We are old. Are you?", or "We are men. Are you?"
you are waiting for some friends to pick you up, someone knocks on the door and you ask "siz misiniz". Is that a possible interpretation?
Yes, it is possible. However while asking "who is it" on the door, you can say "Kim o?" or "Kimsiniz?". These two are more common.
Could you explain what it means
So in English, you have to always have a subject. In this sentence, there is a dummy subject which doesn't really have any meaning unless there is a context. "it" can mean anything you want in this sentence.
So this is right
İs that him
'Is this you?'
Would the question be correct/complete if we just ask 'misiniz'?
Misiniz only would be incomplete . Misiniz isnot a word. The
Question form is not a separate word, in spoken language it is one word (sizmisiniz) but written separately