By now we've all seen the photo of the very large passenger sticking into the aisle of an American Airlines single aisle plane. Was it real? Some kind of Photoshopped hoax? How could they even have let him on board?

Many airlines now have some kind of language in their travel info pages concerning when and if a passenger may be required to buy an extra seat, or may be so large as to be denied boarding. But others don't make this information easy to find, or haven't quite made up their minds about how to handle this weighty topic.

Continental, for example, states that,

"A customer flying in the economy cabin who is not able to safely and comfortably fit in a single seat is required to purchase an additional seat for each leg of their itinerary. The second seat may be purchased for the same fare as the original seat, provided it is purchased at the same time. A customer who does not purchase an extra seat in advance may be required to do so on the day of departure for the fare level available on the day of departure. The customer may instead choose to purchase a ticket for First Class/Business/BusinessFirst®, or elect to pay for an upgrade to the front cabin if there is availability to do so. Continental Airlines is not required to provide additional seats or upgrades free of charge.

A customer is required to purchase an additional seat or upgrade if they do not meet one of the following criteria:

1. The customer must be able to properly attach, buckle and wear the seatbelt, with one extension* if necessary, whenever the seatbelt sign is illuminated or as instructed by a crew member.
2. The customer must be able to remain seated with the seat armrest(s) down for the entirety of the flight.
3. The customer must not significantly encroach upon the adjacent seating space. See our seat maps.

Continental Airlines will not board a customer who declines to purchase a ticket for an additional seat or upgrade for each leg of their itinerary when required."

Southwest Airlines says that the armest is the "definite boundary" between seats, which measure 17 inches in width. You're supposed to book two seats if you think you won't be able to live within those bounds, and if it turns out all the seats didn't sell on your flight, you'll get a refund for the additional seat. Seems fair. In fact, Southwest says that 97% of passengers end up getting a refund.

United has similar rules.

Perhaps what's needed are "sizer" seats near airport check in desks, just as airlines have sizers for carryon luggage.  "If you can't fit into this seat, you need to buy two of them."