Carry-on luggage limit enforcement authority

The airline can and does enforce carry on baggage limits, it is not a duty relegated to TSA. Actually TSA couldn't care less, as long as your items fit through their scanner.

How strict the rule is enforced is determined by realistically just two factors: 1) the plane you are boarding; 2) the level of due diligence engaged in by the gate agent.

As an example of plane based issues, based on your avatar, flying out of Gainesville frequently means flying on Canadair regional jets, whose overhead baggage compartments are minuscule and whose under seat space is also small, so there your screen's box might not fit either and would have to be gate checked.

From a gate agent point of view, they are technically responsible to make sure people boarding obey the rules, though usually they are busy with other aspects of boarding so don't do carry on bag checks on every flight, but I have seen them check.

I have seen JetBlue in Ft Lauderdale enforce carry on size restrictions prior to going through security.

They did this by having Jetbue staff at the entrance to the security line get passengers with suspect hand luggage place said luggage in those metal framed devices that indicate the maximum carry-on size (the ones that people always ignore).

I believe that JetBlue was doing this because it was using an older/smaller aircraft with much smaller than typical overhead luggage bins. This invalidates your contention of

I suspect the airline would be indifferent

As in the case of JetBlue they were not indifferent to carry-on size. Thus the airline and type of plane you will be boarding will be decisive factors in the carry-ons allowed to be carried on.

1. How strict are the US authorities. The "authorities" are the airlines themselves. They know what are the limits of their equipment and they optimize how much space is used to carry passengers, luggage, and freight.
2. would it be the TSA No. Luggage size and weight limits were enforced decades before the TSA was invented.
3. I suspect the airline would be indifferent. It is the airlines who have the exclusive interest (and authority) to establish and enforce luggage size and weight limits. This is both an economic and a safety issue.

Different airlines have different limits, and some have different limits depending on which equipment is being used. If you regularly fly through certain airports or on certain carriers, then it is your responsibility to select luggage that fits their limits. Or else be prepared to check it in for handling by the luggage primates.

There are airports in the world where the carry on size is enforced by security somewhat because of the trays employed -- Heathrow and Budapest comes to mind -- but the United States does not have such idiot systems. Whether the airline cares or not is hard to say but it is indeed likely you will get away with it, for example the United sizer is 10"x15"x23" somewhat bigger than the size in their regulation. This is not an endorsement to take a 15" super-duper fragile item on a flight. I am just giving free advice and can not be held responsible over what happens.

My experience with JetBlue and their 22x14x9 carry on size has been they are very lax with enforcement. Especially with width and depth. Height remember may impact you when you try to get the bag in their bins.

They've taken a more passive aggressive approach of asking for folks to volunteer carry-on to be checked for free especially when flights are full. An effort to speed up boarding and minimize need for messy possibly confrontational enforcement. I once took advantage and had a heavy carry on of filled with clothes checked and asked that it be transferred to my connecting flight. free. For an expensive flight I usually add 1 checked bag for points. Takes the pressure off too.

-Will