If you still haven't been able to secure that empty seat, try again at the gate by speaking with the gate agent. At this point, they should know almost exactly how many people will be flying and what the final seating arrangements will be. Some aircraft have a few seats that are larger than normal. Ask when making your reservation if there are such seats and request one of them. Even a single inch of extra room can make a dramatic difference.
Some oversized persons prefer the window seat.
Others, who are especially wide through the shoulders, prefer the aisle. Wide-shouldered persons can lean out into the aisle most of the time, moving back out of the aisle when traffic comes through. Aisle seats also allow room for long legs to encroach a little into the aisle. Preboard Airlines usually allow passengers with special needs to board ahead of the rest of the passengers. You can often join that group and get seated before the crowd rushes in.