When I was speaking to the flight attendant on my flight where I only had 2/3 of my seat, she told me that as long as a person can lower the arm on the seat they don't have to purchase 2 seats. I probably could have complained before we took off but as there were no empty seats I was afraid they would have made me take a different flight as the woman sitting next to me was already in her seat. I really didn't want to do that and I would have felt bad trying to make the airlines kick off the passenger next to me.The armrest is the diffinitive boundary on most airlines so what the flight attendent told you is correct. Most airlines go by the armrest rule . In fact Southwest Airlines actually publishes it on its' website.
Following copied directly from Southwest Airlines Website:
Customers of Size Policy
Guidelines for Customers of Size Link directly to website http://www.southwest.com/html/custom...index-pol.html
Customers who encroach upon any part of the neighboring seat(s) should proactively book the needed number of seats prior to travel. The armrest is considered to be the definitive boundary between seats and measures 17 inches in width. The purchase of additional seats serves as a notification to Southwest of a special seating need. Most importantly, it ensures that all Customers onboard have access to safe and comfortable seating. You may contact us for a refund of the cost of additional seating after travel, provided the flight does not oversell (which means having more confirmed Customers than seats on the aircraft).