A typical economy class aircraft seat is about 17 inches wide.
With more than one-third of US adults considered to be obese according to an article on the CDC (Centers for Disease Control and Prevention) website, many airlines have introduced special rules for their so-called “customers of size.”
These rules require the purchase of an extra seat by travelers who can not fit into one economy seat – without lifting the arm rests.
Many airlines also reserve the right to refuse boarding to such passengers if an extra seat is not purchased. However, some airlines – such as Alaska – refund the cost of a purchased extra-seat on below capacity flights.
A handy 2012 guide to the “customer of size“ policies of various airlines can be found here:
Airline “Customer of Size” Policies