Travelers who are larger than the average size should research an airline's "Customer of Size" regulations before booking a seat on a flight and check SeatGuru.com for information on which airlines offer the larger seats and which seats offer the most space. SeatGuru.com contains seating plans for most commercial flights.
The U.S. Federal Aviation Administration mandates that a passenger must be able to sit in an airline seat with the armrests down and seatbelt fastened to travel on a flight. Check the airline's "Customer of Size" policy to see whether the airline supplies belt extenders, as passengers can no longer use personal belt extenders as of 2015.
Some airlines, such as American Airlines and Southwest Airlines, require a customer to buy a second seat if she is unable to fit in a single seat. If the flight is full, airlines such as Delta Airlines can ask an overweight passenger to wait for the next available flight and book an additional seat. Purchasing a seat upgrade during the booking process normally guarantees additional seat width and leg length.