Right, I should have said that I measured it, and it’s an inch too big in each direction, but it won’t have much in it and can be scrunched down to fit.
As long as it scrunches and fits completely under the seat then, I know in the past, we did not have a problem. It's can't be sticking out by the feet at all.
There should be devices close to check in or the security gate when you can put the bag in to see if it will fit.
We have always taken our full size backpacks on Spirit without a problem and I'm sure at least a few of them were technically too large. If you can fit it under the seat there won't be a problem.
Right, I should have said that I measured it, and it’s an inch too big in each direction, but it won’t have much in it and can be scrunched down to fit.