As I understand it, the United States limits are 22x9x14 inches (559×229×356 mm). I have a box for my computer monitor box that is 22x3x15 in (559×76×381 mm), so it does not comply and I would expect it to be forced into check-in. I would prefer to carry it by hand: I would expect it to fit in the overhead compartment because of compliance with the length and thickness compliance (within 22X14 footprint). It's foot print does not comply with the footprint under a seat.
How strict are the US authorities with the 14 inch (356 mm) limit?
That being said, would it be the TSA that would that would force the issue/prevent me from boarding with a 22x3x15 in (559×76×381 mm) monitor box?
I suspect the airline would be indifferent. If you have had success in security screening an item (bag) that exceeded the limits, please state this in your response / comment. I am not inclined to 'play this to the edge', however, I would like to understand the current practice