What size underseat bag fits most U.S. airlines?

Most major U.S. carriers accept a personal item around 18x14x8 inches, which is what Spirit and Frontier enforce most strictly. Bags built to 17x13x8 or smaller (like the Travelpro Maxlite 5 tote) tend to clear under-seat space on regional jets too, while 16x14x9-ish rolling underseaters can be tight on smaller aircraft. Always check the specific airline's personal-item dimensions before buying.