I flew twice this month, both on Southwest. I have to say that whether or not you'll "fit" depends on a number of things, one being how your weight is distributed.
I have "thunder thighs" and a big ole booty
This means I "spill over" into the other seats and the arm rest will NOT go down.
Southwest has a clause online about "customers of size" that states that if the arm rest does not go down, the customer may be required to purchase 2 seats, the cost of one which will be reimbursed if the flight is not full.
Like many, I fly with family or friends most of the time and choose the aisle seat to allow my saddlebag on one side some space as I cozy up to my friend or family next to me in the middle
For the first time ever, I needed a seat belt extender.
The unaltered seat belt *did* fit but kept coming open, so I bit the bullet and asked for an extender and was glad I did, as it was more comfortable that way.