I've only once had a problem with carry on size - and that was ex LHR when the contract agent at check-in was insistent that my wheeled suit pack was too big (which it technically is) but everytime I travel in J, I take it and have never been questioned. I was in Y this time (although used miles to upgrade) and I couldnt be bothered waving my arms - so I checked it. Despite all that, it fits in every 747/777/A380 overhead with ease...