Because many different airlines operate a slate of different planes, there isn’t a standard under-seat luggage size, although there is an average: 16 inches x 12 inches x 6 inches. Some airlines allow personal items larger than this, but you should consult with their customer service for specifics. Our favorite personal item-sized travel pack was the Timbuk2 Never Check Expandable Backpack, which at 24 L compressed easily slides under a seat.