95b323b7_of_Size__Policies___Yahoo_News__Stated_policy
[Airline] Frontier [Costs] The seat will be offered at the lowest possible fare at the time of purchase. Refunds will be available for those who purchase a seat and find that their flight has one or more open seats. [Stated policy] 
If staff determines that you will not fit into one seat, it can require you to purchase one.
Airline "Customer of Size" Policies - Yahoo News
Stated policy