One free personal item per passenger is permitted on Frontier Airlines; however, it must not be more than 14 x 18 x 8 inches (35.5 x 45.7 x 20.3 cm), including handles, wheels, and straps. It can be a backpack, laptop bag, or pocketbook, and it should fit beneath the seat in front of you.
https://airlineterminalservice.....com/blog/frontier-a