The hand baggage allowance on Ryanair flights in 2013 is 55cm x 40cm x 20cm (21.6" x 15.7" x 7.8"). Note that many bags that are labeled as 'carry on' are in fact too big for Ryanair.
See how Ryanair's hand baggage allowance compares: Hand Baggage Allowance for European Airlines
If your bag is too big, you will pay a fee, and sometimes even when your bag is fine, Ryanair may still charge you. See the surprising pictures: How Strict is Ryanair's Hand Baggage Allowance?
The IATA (International Air Transport Association) standard hand baggage allowance (followed by and most other airlines (including Jet2 and British Airways) is 56cm x 45cm x 25cm (22" x 17.7" x 9.8") (see the link above for more on other airlines' hand baggage allowance.
easyJet also has the IATA standard (56cm x 45cm x 25cm), as well as a new 'guaranteed cabin baggage' size of 50cm x 40cm x 20cm (19.7" x 15.7" x 7.9") from July 2, 2013. With overhead baggage rack space tight on budget flights, most airlines have started taking some people's cabin luggage off them and checking it into the hold (for free) if there isn't enough room in the cabin. easyJet is the first airline to address this issue: take a smaller case and you are guaranteed to be able to keep it in the cabin; take the standard size and you might need to check it into the hold.
An unusual solution to fitting all of your luggage into your cabin baggage allowance is this Big Pocket Travel Vest, which has pockets large enough to fit a laptop. Ryanair's CEO, Michael O'Leary, says the airline will not charge for these jackets, and it is hard to see how he would be able to.
If you're in the market for a more conventional case, check out the ones below.
Carry-On Suitcases for Ryanair
All the cases below are appropriate for all flights in Europe, including Ryanair and Wizz Air. If flying with any other airline, including easyJet, you can actually take a larger case.
Note: The cases on this page are for delivery to the UK and some other European countries only. US and Canada readers, click here: Ryanair Hand Luggage.
Dimensions: 48.5cm x 36 x 20 cm Weight: 2kg
A famous and reliable brand, Timberland make a great choice for Ryanair hand luggage. Comes with a combination lock and a document pocket.
Dimensions: 50 x 38 x 20 cm Weight: 3.5kg
Incredibly light bag, though the odd shape means you lose a lot in capacity. Remember, you're only allowed 10kg in total in your Ryanair hand luggage. Comes with Knomo ID tracker system to help you find your bag if lost
Dimensions: 49 x 30 x 20 cm Weight: 1kg
Though this bag is smaller than some of the others on this page, its reasonable weight is a real plus, while its shape doesn't hold it back (unlike the Knomo).
Dimensions: 45 x 34 x 20 cm Weight: 2.4 kg