Here's the official criteria from the Southwest website:
"The armrest is the definitive gauge for a customer of size. It serves as the boundary between seats, which measure 17 inches in width. Customers who are unable to lower both armrests and/or who encroach upon any portion of the adjacent seat should proactively book the number of seats needed prior to travel."
So the armrest is the "definitive" gauge, yet "encroachment" is the big no-no? Hard to figure if passengers like Pau Gasol of the LA Lakers (7', 250 pounds) or LeBron James of the Miami Heat (6'8", also 250 pounds) would have to buy one seat or two. It sounds like just one, since their "overflow" is vertical, not horizontal. But say you're stuck in a middle seat between these two giants; surely you would feel some sort of encroachment in the shoulder area, right?
Guess again: "Simply having broad shoulders would not necessarily prevent another customer from occupying the adjoining seat. The upper body can be adjusted, but the portion of the body in the actual seating and armrest area doesn't have this flexibility," the Southwest website states.
Question: will your broad-shouldered seatmate cooperate with upper body adjustment? They may have trouble moving at all. Imagine Gasol or James in coach seats; the poor guys would have to pass up the free Cokes because I can't see how they'd get their seatback tray tables down. Sure is different from NBA chartered flights with so-called "rock star configurations": news reports say they get 60 inches of space between the back of one seat and the front of another (while mere mortals in coach get about 30 inches of space).
Of course, most players will spring for first class, or hitch a ride on a friend's jet (I've read that Kobe Bryant has a pretty nice one), but you'd be surprised how many big-time celebrities have flown Southwest, including Brad Pitt, Lady Gaga, Venus Williams, Justin Timberlake and more.
For those who are not celebrities, though, the biggest problem is the inconsistent application of the "customer of size" policy. Last April, for example, flyer Kenlie Tiggeman was about to board a Southwest plane in Dallas when she was told she needed to purchase a second seat; yet, on her Southwest flight to Dallas, no one said a thing.
By the way, I am not slamming Southwest; this popular airline flies more people in the U.S. than any other carrier, and the fact that it's the only airline that allows passengers two free checked bags is one of the reasons it's so beloved (though it does seem as if Southwest attracts the most vociferous Twitter users).
Several other carriers also have "too fat to fly" policies including Alaska, American and United, and they have these policies because other passengers are sick of not always getting the entire seat they paid for.
It's a delicate balancing act (Southwest ultimately apologized to flyer Tiggeman), but these policies will not go away, so be prepared.
You have two choices. You can be proactive and buy two seats to avoid a potentially embarrassing scene at the gate, or gamble that you will fit in a single seat. Most, I suspect, will choose the latter, since there isn't much of an incentive to pay in advance for a second, not when airlines like United say they will make every attempt to larger passengers next to empty seats; hey, hope springs eternal.
However, if you lose your gamble and there isn't an extra empty seat on your plane, you could be locked out of your flight altogether, and you don't want that. Just ask the NBA players.