Any person who is unable to lower the armrest or encroaches on the adjacent seat is considered a passenger of size. Most major airlines have policies about this.

While some airlines may be able to accommodate you last minute at the airport, it is a good idea to contact the airline directly and purchase a second seat to verify seat inventory and ensure your comfort.  Depending on the airline and how booked a flight is, you may be entitled to a full or partial refund of the second seat.  

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