Seat size surcharges

(CNN) - Thanksgiving kicks off a holiday season full of packed flights -- and this year some airlines are enforcing new policies for passengers who can't fit in a standard seat, meaning overweight flyers face possible extra fees or being prevented from boarding the plane.

Website says on some airlines, passengers who can't squeeze into the standard economy seat may have to buy another seat.

Different airlines have different approaches in accommodating overweight passengers. Air Canada, for example, looks at obesity from a medical standpoint - they allow overweight passengers a free extra seat if they have a doctor's note. Without a note, they must buy a second seat.

According to Airfare Watchdog, airlines like American require overweight passengers to buy an extra seat if they can't buckle their seat belt with an extender, lower arm rests or if they are encroaching on the neighboring passenger. Carriers like Delta don't require an additional seat purchase but they can ask a passenger to board another flight if space is limited.

For Southwest, customers who encroach on their neighbor's seat are forced to buy an additional ticket. If the flight is not overbooked, the passenger is refunded.

In most cases, these rules are often observed on a case-by-case basis.