95b323b7_of_Size__Policies___Yahoo_News__Stated_policy
[Airline] Southwest [Costs] The seat will be offered at the same price as the first one at time of booking it is not recommended to wait until you arrive at the airport; Southwest is known for being particularly inflexible on this issue. Contact for a refund of the cost of additional seating after travel, provided the flight is not entirely full or has not oversold. [Stated policy] 
Customers who encroach on any part of a neighboring seat should proactively book the needed number of seats prior to travel. (The armrest is considered to be the definitive boundary and measures 17 in width.)
Airline "Customer of Size" Policies - Yahoo News
Stated policy