You should fit fine.
Southwest did recently change their policy however and if you ask at the gate for an extra seat due to your size, it will be provided free of charge.
My BF and I love it! I get the aisle, he gets the window, and no one is in between us! Plus I can lift the armrests and scoot over and snuggle onto his shoulder to sleep lol
We fly Southwest all the time and I am a size 26/28. I have only been made to purchase an extra seat once, last June. It was very embarrassing, and the staff member was very rude. She told me that if I wanted to see a copy of the Customer of Size policy she wouldn't let me onto my flight! Needless to say we purchased the seat, and when we landed we called the 1-800-I-FLY-SWA line and filed a complaint. They comped me an extra seat on the flight home (at the time you still had to pay for them), refunded the cost of the extra seat on the flight there, and gave us $150 in credit that we are using towards our flight in 3 weeks.
I am VERY GLAD that they changed their policy, and look forward to my FREE extra seat this time around!!