The worst they can do is tell you they can't help you. Arrive early for your flight and make your request when you check in at the ticket counter. At that point, they may have a feel for the flight's seating and may be able to help you. 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.