Even if someone has used one, you could have a totally different experience.
If they had a crew of Spirit workers who didn't care or were having a great day, they may have been able to use something that isn't typically permitted.
If you get a grumpy, hate the world crew, they could measure every bag that goes through.
You need to measure the bag and be certain that it complies with the dimensions required by Spirit.
That is the only way you will know for certain that it will be permitted, and the only way you will have a leg to stand on if they challenge the size.
As a pp suggested, either go to the airport prior to your trip and make sure it fits in the sizing bin, or pack it in such a way that you still have the ability to squish it down to size.