Article from TSA Travel Tips

Maximum Dimensions for your Carry-On Luggage by Domestic Airline

United Airlines

  9 inches x 14 inches x 22 inches (22 cm x 35 cm x 56 cm), including handles and wheels. (may not exceed 45 linear inches)

American Airlines

22 inches x 14 inches x 9 inches / 23 x 36 x 56 centimeters (including handles and wheels) (may not exceed 45 linear inches)

Southwest Airlines

10 inches x 16 inches x 24 inches (may not exceed 50 linear inches)

Alaska Airlines

24 inches x  10 inches x 17 inches (61cm x 25cm x 43cm), including wheels and handles (may not exceed 51 linear inches)

Delta Airlines

22 inches x 14 inches x 9 inches (56 x 35 x  23 cm) (Carry-on bags may not exceed 45 linear inches in combined length x width x height)

JetBlue

22 inches x 14 inches x 9 inches, including wheels/handles (may not exceed 50 linear inches)

Hawaiian Airlines

9 inches x 14 inches x 22 inches  (may not exceed 45 linear inches) Weight limit of 25 lbs per passenger (the only airline on the list with a weight limit for carry-ons)

Spirit Airlines

22 inches x 18 inches x10 inches (may not exceed 50 linear inches)

Virgin America

24 inches x 16 inches x 10 inches (may not exceed 50 linear inches)

Allegiant Airlines

22 inches x 14 inches x 9 inches (may not exceed 45 linear inches)

Horizon Air

24 inches x  10 inches x 17 inches (61cm x 25cm x 43cm), including wheels and handles (may not exceed 51 linear inches)

Frontier Airlines

24 inches x 16 inches x 10 inches (may not exceed 50 linear inches)

Sun Country

24 inches x 16 inches x 11 inches (may not exceed 51 linear inches)