Four to 6 ounces of pork per guest is the rule to follow in determining how large of a roast to purchase. This average is variable based on the type of pork roast you are serving to your guests.
There are many different cuts of pork, and the particular cut determines how many ounces to anticipate serving to your guests. Pork loin is leaner and has more meatier areas per ounce than a fattier pork shoulder or butt. Also consider unanticipated guests in your final roast size. Not having enough protein for your dinner guests is an embarrassing misstep even for an experienced host.