Is it a bag or a carry-on suitcase? I've somewhat frequently carried a full-to-the-brim, barely-closeable gym/duffel bag as hand luggage, and it's significantly more than 3cm too large in two dimensions. Never been had the size questioned. Anecdotal experience says that you're much less likely to have the size challeneged if it's soft bag as opposed to a carry-on suitcase.