Some countries hunt certain kinds of whales, despite the whales being on the endangered list.
My guess is that, depending on the size of the whale, a whale rib would be several feet long (for a smaller whale like an orca which is actually the largest dolphin) to maybe ten feet long or longer for a huge whale. Probably the rib itself is at least several inches in diameter, because it’s got to be strong enough to hold up the enormous weight of the animal, and to not break when diving down with the massive pressure of the deep water. Probably not too easy to cut and so not practical for the average barbecue.