So I will be flying to San Diego for for my surgery in Mexico with multiple planes. Any suggestions on which seat would be best? I know most likely I will need a seatbelt extension. My stats are 5'5 and 315lbs, I have seen most seats are 17 wide and I think I am about 20-22 wide. I don't want to have to purchase another seat.

Isle seat?? I flew there for mine and I didn't really need much during flight. I expected the seat belt to hurt or be uncomfortable on my way back which it was not. Wear a loose top and comfortable pants on your way back.

An aisle seat would be the best. I didn't want to crawl over people to use the restroom. My other suggestion, and I'm sorry if it's TMI, but try be be near a bathroom on the plane home. I had diarrhea from the antibiotics.

I paid extra for the seats with extra room and picked one near the restroom. I haven't gone yet so I don't know it this was necessary but I figure its better to be safe than sorry.

I flew southwest since they don't assign seats. I sat on the aisle. I didn't fit between both lowered armrests, so I kept the one toward the middle seat up behind my back. Fortunately there were unfilled seats, so no one sat beside me. Had the plane been full, I might have needed to take a different flight. (And I did pay extra at the gate for priority boarding).

I flew Southwest, I paid for an extra seat for me, my wife was with me so we had a row of three to ourselves on each leg of the flight, two of the legs were completely full. Southwest is unique as 1. The allow you to pre-board if you have the extra seat so you are guaranteed to get it. 2. After the flight all you have to do is call them and they will refund you the cost of the extra seat. I know it makes no sense why they do that but it is in the Passengers of Size policy (they are the only airline that does this that I am aware of). As soon as I got home I called them and 7 days later the amount was credited to my credit card.

It made the trip so less stressful, if you can use Southwest I highly recommend it.