Cabin baggage rules are clear: 22x14x10. Just because they are ignored in other places it doesn't follow that they will be ignored in HK. There's no point ever debating with those bag checkers - if they call you, just say OK and go back to check-in. If you know you're oversize then best to ask for the tag when you check in anyway.

I believe that the airline has to pay for over size bags through security.