Here in north Alabama, when you buy a BBQ pork sandwich or plate at a "real" BBQ place, the slaw that you typically get (in fact I have never seen the other served at these places) is a vinegar-based cole slaw. Now, if you go to other types of restaurants, then you'll usually get the mayo version but with BBQ, I've only ever seen the vinegar style, which I personally don't like a bit.

Oh, and to answer the OP's question, you should probably get between 4 and 8 ounces depending on the generosity of the food preparer.