Not only will United sell you two adjoining seats, but the carrier has a new policy to give you FF credit for both seats. You have to submit the second boarding pass separately (it's not automatic.)

Even these days, some of the first class and business class seats aren't wide enough for larger people. Delta currently has some FC seats that are 18.5 inches wide - only a half inch wider than the coach seats. Width is usually the biggest issue for large passengers. Legroom is great, but width is critical.

I buy two coach seats if I can't upgrade, although on some (alas, not all) planes, the aisle armrest goes up, which is a HUGE help too.

Of course, if airline seats were only 1-2 inches wider, this issue would be moot for larger people, and those who are smaller would be even MORE comfortable in coach. Everyone would win! Well, except the profit margins of the carriers. Airbuses are more comfortable because the fuselage is several inches inches wider than a 737, so each seat is 1 inch wider. Just think if the fuselage was 15 inches wider! Not enough to cram another seat, but enough to make each seat much more comfortable.