Recently watched a show on TLC called My 600-lb Life and a lady who was 661 lbs. was shown flying on a US based flight on an unmentioned airline (although it appears to be a Southwest plane seat) and who required three seats with a belt extender. If one is of this size or smaller, it would seem that accommodations exist for one to fly.
Here is a screen shot from the show of the lady sitting in and requiring three (3) seats of the plane:
(My 600-lb Life | Erica's Story | Feb 8, 2017 | Copyright © 2017 Discovery Communications, LLC.)