My carry on bag is a duffel bag. It always fits. Even in a little RJ.. I've never ever put it in a sizer, nor have I really ever seen any gate agent force someone to do so or bust out a tape measure and actually measure it. The supposed "max size" is meaningless because I am sure people consistently and regularly exceed whatever so called "rule" is in place. At least, this is my experience on domestic USA flights. But I have not taken a flight to Europe and was concerned they were more strict.

Really, I wasn't sure if it was measured across the diagonal like a TV or of it was how long/high it was.