plunet there are rules for cabin baggage but QR seldom enforces them unless the size/weight is obviously excessive. The 50cm limit has been discussed several times on this forum and the general feeling was that 55cm would normally be acceptable. Do remember that the size of the overhead lockers varies from aircraft type to aircraft type.

Be warned that some airports have their own cabin baggage rules that they strictly enforce.

You may find the http://www.flyertalk.com/forum/qatar...allowance.html thread helpful since it also includes cabin baggage despite the title.