As far as I've seen, the subject is only completely left out like this in third person.
It's more common to leave out the verb in these sentences, like "mimi mnene", "sisi wanene" and so on.
Great is pretty ambiguous in English and although it's original meaning is "large", I'd generally interpret that as "very good".
Because "wanene" is plural and refers to people or animals (not a single, inanimate thing).
I don't think it entirely matters whether you are likely to use the exact phrase, building an understanding of the construction is helpful, for which I really appreciate other users explanations. I guess the most likely might be in a discussion of livestock and whether they are ready for market - "are they fat?" seems like a possible question.
Well, I haven't had much use for the sentence "Police is not a Politician" either up to now, but "Are they large?" seems a lot more useful and at least grammatically correct. It is a good example of adjective agreement.
For a realistic context, what about:
"My brothers eat a lot." - "Are they large?"