95b323b7_of_Size__Policies___Yahoo_News__Stated_policy
[Airline] Allegiant [Costs] Seat can be purchased at same price as the first in advance; otherwise lowest available fare at time of booking will be offered. If open seats are available, there is no charge for a second seat for those who want to wing it however, no guarantees and no compensation if you are bumped. [Stated policy] 
Customers must fit in the 17.8 space between armrests; if seated next to friend or family, fine, otherwise second seat required.
Airline "Customer of Size" Policies - Yahoo News
Stated policy