This question has been asked before.

http://www.flyertalk.com/forum/trave...sengers-y.html
http://www.flyertalk.com/forum/trave...t-what-do.html

Nonetheless, here's AA's policy:

http://www.aa.com/i18n/travelInforma...false&from=Nav


Customers Requiring Extra Space

For the safety and comfort of all customers, we have aligned our seating accommodation policies with other major airlines regarding customers who may require more than one seat because they are:

* Unable to fit into a single seat in their ticketed cabin and/or
* Unable to properly buckle their seatbelt using a single seatbelt extender (available upon request from a flight attendant) and/or
* Unable to lower both armrests without encroaching upon the adjacent seating space or another customer.

Reservations

We encourage customers to address their seating needs when booking the original reservation. If you meet the above criteria, one of our Reservations representatives will ensure that you receive two adjacent seats at the same rate for each seat.

At the Airport

If you meet any of the above criteria and have not booked an extra seat in advance, please see an airport agent. If seats are available in your ticketed cabin, you may be accommodated in the same cabin next to an empty seat. The additional seating must be available without downgrading or unseating another customer. If time allows, and upon payment of the fare difference, you may be offered a seat in a higher class of service that may provide more space*.

If additional seating is not available, you may purchase a second adjacent seat on a different flight. The fare for the second seat will be the same as the original seat.