The TEMPLATE would decide the correct size of ALL BAGS at all airlines. How consistent is that as you go through security, you're "carry-on" bag MUST fit through the template. No cheating. It fits or it doesnt.
Try arguing with a TSA agent as compared to a flight attendant. I dont fly the Lead/senior/purser position because I refuse to deal with passengers who wont check their bags because the overheads are full. Last thing I want is to take that bag away, after a confrontation and end up spending the next SIX HOURS with that individual who's pissed off at me because the bins were full. I like how they blame me that everyone brought onboard bags and they couldnt. Keep the air rage on the ground and keep the oversized bags from getting past security.
Number of bags and the SIZE OF BAGS should be a federal law. It's a safety issue, not a competitive one where airline management decides what size of bag should be permitted.