This video shows what you can fit inside Spirit's 18x14x8 bin as your free personal item. UPDATE: Speaking generally, Spirit may not make you measure your bag at the gate. If you bag looks like it would fit, they usually let you pass. Allow this video to be a reference to what will absolutely fit in their bin, and then the choice is yours as to how much bigger of a bag you'd like to try to get on a Spirit plane.
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