I agree with the other comments saying not to worry about asking for an extender. Frankly, anybody who looks down on you for being fat wouldn't change their opinion based on whether or not you have an extender. So really nothing to worry about.

However, as noted, do expect the seats to be very cramped. And as you are an infrequent flyer, be sure to read up on Spirit's additional fees. Not being prepared could cost you a lot of money.