"My son is a big doctor."
Translation:اِبْني دُكْتور كَبير.
actually.. it is so but in dialectical form. Duolingo is mixing proper Arabic with dialects. Kabeer do mean all what you've said in some dialects but not in proper Arabic.
Unless they do indeed talk about size (which makes no sense).
I would put that as (my son is a famous doctor) ابني دكتور مشهور