I would assume that those airlines charging for hold baggage (Easyjet, Ryanair, Flybe etc) will try and catch passengers out. Because passengers (understandably) do not want to pay over the odds to check in a suitcase, their hand baggage often ends up being much larger than it should be. I fly a lot, but rarely low cost or charter, and have never been quizzed over the size of my hand baggage, which kind of proves my point. Flying the major, full-fare carriers, I cannot see you having much a problem if the dimensions of your carry-on bag is larger than what the airline supposedly allows.