IMO, no smaller than a 180 and even that is way too small IME.
I would not put them in a reef tank, no way.
First, they will eat crustaceans.
Second, they are bulldozers, and will climb there way up and through rock work, including over corals.
Third, they are big eaters, so water quality suffers - which is not what you want with sharks.
IMO, they should only be kept in species tanks, where they are the centerpiece, and all fish/decor is selected with them as the primary consideration.