We frequently get asked:
What are the maximum dimensions and weight you can take as hand baggage on airlines?
With all the current security measures now in place, each airline has its own size and weight allowance for carry-on, although usually only one bag is allowed as carry-on.
So, we have compiled a list of the current* Airline carry-on allowance for some frequently used airlines departing from the UK:
Airline Hand Baggage Allowance
*carry-on allowance at 8th July 2010, as stated on the Airline websites
Snapperstuff.com Blog 
An update for the chart on flybe’s hand baggage allowance. Their page on baggage http://www.flybe.com/flightInfo/baggage.htm clearly states the maximum dimensions 50 x 35 x 23cm. (I can vouch for this size and it being slightly smaller than the average as I travel with them regularly; their fleet is generally smaller prop aircraft with small luggage bins overhead.)
Regards