Typically, a carry-on bag should not exceed 22 x 14 x 9 inches (inclusive of the handle and wheels). However, the rules can vary by airline. For example, American Airlines' carry-on luggage size is 22 ...
Tucked away in a recent Alaska Airlines blog post with travel tips for carry-on luggage is a notification that the airline will reduce the acceptable maximum dimensions for carry-ons starting in June.