I fly Spirit quite alot, being in Myrtle Beach. They are pretty strict on the size of the "personal" free carryon. In Myrtle beach airport they are not as fussy,but if you're flying into or out of Boston or N.Y. they are FUSSY. If the duffle can be squished down to those measurements than you should be all right. I bring a small roll on exactly the correct measurements and a BIG pocketbook. Don't pay for seats, you'll be assigned 2 together if you're on the same locater#.The bigger roll ons that you see people bringing on...they've probably paid to bring on beforehand.