As of early 2016 the US/Canada airline industry did not regulate the person of size seating. There are no standard definitions, nor terminology, nor regulations for persons of size seating. This is surprising considering the prevalence of the issue and the potential for negative social media postings. Only time will tell if the airline industry will address this increasingly frequent issue. Here is what the FAA, U.S. DOT, ADA have to say about person of size accomodation.
The U.S. Department of Transportation (DOT) has confirmed that airlines are not required by law to provide additional seating or an upgrade in the event additional space is required for a passenger. [source: United Airlines]
Title II of the Americans with Disabilities Act (ADA), Section 12141(2), specifically excludes airline travel. Airline travel is instead, covered by the Air Carrier Access Act (ACAA), 49 U.S.C. 1374(c) and regulations implementing the Act issued by the Department of Transportation as 14 CFR Part 382. The ACAA preceded the ADA, and Congress excluded air carriers and other air transportation services from the scope of ADA. [source: Southwest Airlines]
As regulated under 14 CFR 382.87: Seating accommodations (f) “Carriers are not required to furnish more than one seat per ticket or to provide a seat in a class of service other than the one the passenger has purchased.” [source: Southwest Airlines]