True, but your target leaves way too much room for interpretation and subjectivity, and opinion. How do you define "unreasonably?" Whatever your definition is, the gate agents will have a different definitions, and we're back to square one.

If it fits, it fits. If mine fits in loosely and yours fits in snugly, then we both should be able to board with it.

As I've said on other forums, for some odd reason, my bag always appears bigger to agents, and they are certain it won't fit. I volunteer it for the sizer-bin, and it goes in (snugly, but absolutely complies). At that point, I've been lucky that they have always allowed me to board with it. But I keep reading here that others' bags fit snugly as well, and the GAs insist that it's too big. Totally not cool.

Speaking of not cool, I feel that guitars on board need their own forum for us to complain about. It's ridiculous.