Most have the same overall dimensions but in terms of weight EasyJet stands out for not having weight restrictions on hand luggage in comparison to Air Berlin, which only allows 6 kg.
EasyJet: A standard piece of hand luggage with dimensions not exceeding 56x45x25 cm.
Vueling: Each passenger may carry 1 piece of hand luggage weighing up to 10 kg, with dimensions not exceeding 55x40x20 cm.
Air Berlin: The hand luggage must not weigh more than 6 kg (with laptop: 8 kg) and measures may not exceed 55x40x20 cm.
Ryanair: Each passenger (except infants) may carry a piece of hand luggage for free. “Piece of hand luggage” includes handbags, briefcases, laptops, shopping bags, cameras, etc. This piece must not weigh more than 10 kg and not exceed the maximum dimensions of 55x40x20 cm.
Air Europa: Each passenger is allowed to carry one item of maximum weight of 10 kg in Economy Class and the maximum dimensions permitted are 55x35x25 cm or the sum of length + width + height not exceeding 115 cm (45 in).
Spanair: Is it possible to carry one item of hand luggage whose measures cannot exceed 50x40x25 cm (sum of the 3 measures should not exceed 115 cm) and a maximum weight of 10 kilos for Traveler class.
Aer Lingus: The hand baggage will be limited to a package of up to 10kg per passenger. The maximum dimensions are 55x40x20 cm.
As you can see, the width of allowed hand baggage varies depending on the airline up to 10 cm. It is also important to note that with low cost airlines the allowed weight for both checked baggage and for hand luggage is different, so you should consult it on their websites, since they are extremely strict with the weight and often charge very high billing rates for each kilo of exceeded weight.