I flew United on a 26 hour journey earlier this year, and honestly I only JUST fit in one seat, like I actually dont understand the logistic of how anyone even an inch wider than me would fit with the arm rests down, but apparently they do
and i was only about 260 lbs at the time, so if you are flying with united i would definitely check up on seat guru to see what your seat is going to be like and get there early and ask politely if you can be seated with a spare seat, but beware united does have a policy that states if you cant lower the armrests or require more than 2 seatbelt extenders they can ask you to purchase a second seat.
also just for the record if you ever get a chance to fly Qatar choose them ! the seats were so much bigger and actually comfortable, also always try and get an aisle or window seat rather than middle, but im sure most people know that