Frontier Airlines allows personal items up to 14 inches high by 18 inches broad by 8 inches deep (14" x 18" x 8"). Travelers should measure their bags in advance because anything larger than this is regarded as a carry-on and will incur additional fees. Although Frontier is renowned for its inexpensive base rates, additional fees apply for checked and carry-on baggage. https://airlineterminalservice.com/blog/frontier-airlines-personal-item-size/