In a disappointing move, Spirit has decided to reduce the size of the allowed, free personal item. Here's a snapshot of their website's current guidance:
They've added two inches to the height (or width) of the allowable size, but reduced the depth by four inches. Part of what makes this a silly, insulting change is that it's not as if their seats are going to be lower to the floor of the plane. The allowable depth of the bag should be determined by what fits under the seat, as that is exactly where you'll have to put your personal item. (Dissenters may point out that once on the plane, you're allowed to use the overhead bins for your carry-on. This is correct, but not really in keeping with the "spirit" - heh - of the rule.)
How do other airlines compare?
As it happens, I fly Frontier more frequently than Spirit, and it turns our Spirit is just catching up (or down) to Frontier when it comes to baggage size. Here's a comparison of the personal item allowances at the lower fares for select airlines. The airlines that allow a free carry-on (larger than a personal item) are noted as well.
The change is disappointing mostly because Spirit appears to be moving downwards to catch the competition. Yes, it's true that up-charges are a large part of how Spirit does business, but it's also true that the argument for ever using an ultra-low-cost carrier relies on being able to work around those up-charges.
Insofar as Spirit was looking at American and United for guidance, they've made a crucial mistake. You know who flies American and United and can use their basic fares? Business travelers who take short trips. You know who doesn't fly Spirit? Business travelers. United's basic economy allowance may look ridiculous until you recognize it will fit most small laptop bags and not much else.