TIS, also ask for the first cut when you order the meat. It may be a little more, but it's worth it. Most of our local supermarkets have had sales on rib roasts this week, but only the center cut, which is really fine, but not as good as the first cut.

I usually estimate half a pound of meet per person, especially since a rib roast has heavy bones. If I were cooking for 13, I'd look for a 7-8 lb. roast.