Hi sandy, if you are looking for a bag for hand luggage I found the site below useful. It gives you the sizes certain airlines permit and they sell bags within the smallest size the 20cm one. Most bags on sale seem to be larger than this but they sell a number within this size range and they are not too expensive.
Be very careful - Ryanair are very strict on weights and measurements. The cage that they use to measure looks a bit measly compared with other carriers. If it looks too big, they will make you put it in the cage at the departure gate - and if it wont fit in it gets put in the hold and they will charge you God alone knows how much. Just follow the rules and regs to the letter and you will be fine - we've flown with them several times and had no probs at all.
I agree with LyndaB, though have found some airports stricter than others. Useful tip: use luggage strap to make sure hand luggage fits into cage. Also wear jacket with large pockets - may not look cool, but mine takes book/food for plane and other stuff which allows more room in case. We often travel with just hand luggage and you'd be surprised how much can be crammed in!