Restaurant menus often advertise pieces of rib slabs such as ¼, ⅓, ½, ¾ slab. The term slab, though, is an indefinite term.
A full-sized slab of pork ribs will have 13 ribs in it, but generally in North America, the most you will see is 11 or 12 ribs in the cut.
Many people consider that the best racks of ribs will weigh 3 pounds (1 ⅓ kg) and under, because that weight indicates that it comes from a young hog.