Depends where you're originating. First, it's not CX you have to worry about - it's the insane ladies prior to immigration at Hong Kong. In short, NO you have an extremely low chance of getting something larger on board. Quickly, if you're originating at a non-HK airport, you don't have anything to worry about. If you ARE at HKG:

1.) Prior to exit immigration, you must funnel through a few channels, each guarded by ladies whose sole job it is is to spot oversized luggage.
2.) They do their job extremely well. When I first moved to HKG last year I thought I could sneak by...yea right. I have waited for strategic moments like trying to wait until they were occupied with someone else, only to have them chase me down into the exit immigration area itself. I have tried switching from the south entrance to the north entrance. Futile! They have eagle eyes, it's rather terrifying actually, they know exactly when your bag is too big and call out to you as you're walking by...then they put it in the bin there to prove it's too big, and make you go back and check.
3.) You have an option - if you're CX/OW elite you can ask the CX gate agents to give you a special orange tag that muzzles the crazy bag ladies at immigration. They put the orange tag on your "oversized" carry-on when you check in, then you go to the ladies at exit immigration, they tear off a perforated part, and you're good to go. They don't turn you around. The CX gate agents will care more about WEIGHT not size, so if your carry-on bag is really heavy unload a few things before you check in. The exit immigration ladies care about size only.