Carry-on baggage must be stowed in the aircraft cabin which limits baggage to a size, weight and shape to fit under a passenger seat or in a storage compartment. Cabin baggage should have maximum length of 22 in (56 cm), width of 18 in (45 cm) and depth of 10 in (25 cm). These dimensions include wheels, handles, side pockets, etc. Carry-on items must remain with the passenger at all times and are the responsibility of the passenger.
Programs: AA Explat (1 more time...), UA former 1K PremExec, DL
I wouldn't be surprised if more people had problems with the depth of sizers and bags rather than the length. Many overstuffed bags will not fit even if they did before, not to mention that many bags simply will not fit at all in the depth dimension.