Guide to 'Carry-On' Cabin Baggage Size
You will need to double check with your airline if your bag will be within the allownce for 'carry-on' cabin baggage size before travelling. Below is a general guide from the 'International Air Transport Association (IATA)' to which most major international airlines adhere (within a few centimeters).
The IATA guidline is... "Carry-on baggage must be stowed in the aircraft cabin which limits baggage to a size, weight and shape to fit under a passenger seat or in a storage compartment. Cabin baggage therefore should be a maximum size of:
Length: 22 in (56 cm) Width: 18 in (45 cm) Depth: 10 in (25 cm)
These dimensions include wheels, handles, side pockets, etc. Carry-on items must remain with the passenger at all times and are the responsibility of the passenger."
Please remember that a number of the European Low Cost airlines now charge for 'checked-in' baggage and their size requirements for cabin baggage often differ. This makes the size of potential 'carry-on' bag more important if you are travelling on a tight budget or are limited on time and don't want to wait for the carousel to collect your 'checked-in' bags at your destination.
So again it is important to check with your airline before travelling.