From Wiki:
The International Air Transport Association (IATA) sets guidelines for cabin baggage/hand luggage/carry-on luggage size. They are not mandatory, however, and individual airlines can and do vary their requirements. The IATA guideline is:
Cabin baggage should have a maximum length of 56 cm ( 22 in ) , width of 45 cm ( 18 in ) and depth of 25 cm ( 10 in ) including all handles , side pockets , wheels etc.
Many network airlines apply this standard but beware when travelling LCC or similar.
Read the conditions of carriage before you buy your ticket, and again, before you travel.