Upvote:1
If you carry both a "small backpack" and a "laptop bag", the gate staff is supposed to count that as TWO items and the larger will be considered a Carry-On even if it also fits in the Personal Item sizer.
However, if the Laptop bag fits in the backpack, and the backpack then fits in the "Personal Item" sizer, you're good. You can take the laptop bag out once the flight departs.
Upvote:5
This video shows the sizer which apparently says a personal item is 16" x 14" x 12". Here's the relevant portion of a frame:
I also found it on the Spirit site and it'll change to 18" x 14" x 8" for Travel April 4, 2017 and beyond. This is free and the "normal" carry on, max size 22" x 18" x 10" is paid.