As of May 2015, the maximum size for carry-on luggage varies by airline, but for most U.S. carriers, the maximum size is 22 inches by 14 inches by 9 inches. Southwest and Virgin America have slightly more generous limits of 24 inches by 16 inches by 10 inches.
Jet Blue allows the largest carry-on bags. Its maximum is 26 inches by 18 inches by 12 inches.
The maximum size for carry-on luggage varies among international carriers as well. Most carriers impose a limit of 21.5 inches by 13.5 inches by 9 inches on carry-on bags. Finnair allows travelers a bit more flexibility, with a maximum size of 22 inches by 18 inches by 10 inches. Avies has the most restrictive limit for carry-on bags, with a maximum size of 17.5 inches by 13.5 inches by 8 inches.