I fly thru the mothership 2x a week with my Pelican 1535 full of camera gear. It is of 'legal' size, but just barely as the OP is looking at. It'll fit in the overhead of every DL bird sans the CR2, where it will fit under the seat, but leaving seatmate with about a foot of legroom space - for which I intentionally avoid the CR2s for their last remaining months unless flying with a family member.

Most cities, my hometown CHA included, you just mention it to the FA that it'll fit on "whatever plane in question" and they accept that you know what you're talking about and let you go down the jet ramp.
However, in ATL, 80% of the time, I have to deal with the GA on a power trip who absolutely KNOWS that my Pelican won't fit on the plane and I MUST gate check it....regardless of what I say.

Again, being full of camera gear (>$15k) and spare lithium batteries, Delta can't put it underneath and they won't insure it....but again, doesn't matter to the GAs in ATL.
So there may have been on 'occasion' that on the way to the gate checked location of dropping off, the pink tag has 'fallen' off and I board with it anyway.

And as mentioned - some international check ins - like SYD, the counter agents are strict not only on size, but weight also.