I was searching for a bag to place inside the cabin to save a few pounds on paying more for checked in bags!! I noticed loads of sellers selling bags or suitcases as suitable for cabin baggage. Well I urge you to check with your airline what is the MAX size you can take as cabin baggage as I found over half of items listed as suitable for cabin baggage to be too large for Ryanair (My top choice of airline). So please check and don't take sellers word that its fine, as a start I have listed two budget airlines MAX size allowed, I will list more as I research them or given by readers of this guide
Ryanair: 55 x 40 x 20 cms
Easyjet: 55 x 40 x 20 cms
MORE ADDED
Flybe: 50 x 35 x 23 cms
Aer Lingus 22 x 35 x 45 cm
British Airways: 56 x 45 x 25 cms

Jet 2 50 x 38 x 26 cms thanks to npl22 for update on size

Air Berlin 55 x 40 x 20 cms

bmi baby 55 x 40 x 20 cms - thanks twincle21 for info

Aer Lingus (Irish National carrier) 22 x 35 x 45 cms - thanks to Fidelmag2006 for info