Good choice, Aegean is great to fly with!
Hand baggage allowance states:

Hand luggage

Each passenger may carry on board the plane 1 item of hand luggage, weighing no more than 8 kilos and measuring no more than 55cm in length x 40cm in height x 20cm in width.

It's on here, scroll down for economy,

http://en.aegeanair.com/travel-information/baggage/epitrepomena-oria-aposkeuonheiraposkeuon/

that's the same as Ryanair but quite a bit smaller than easyjets, 56 x 45 x 25cm

It is a bit smaller, but at least you get 20kg of checked luggage included in the ticket price! I have flown with them about a dozen times in the last 2 years & have never seen one of those 'your bag must fit in here' things either, then again I rarely have much with me, I carry as little as possible! With 20kg checked I don't really need anything else, just my camera for safe keeping.

Ah, thank you for that. I think my plan for now is to get one at the dimensions for easy jet and use it next year as checked in baggage. As you say this year I am only taking a case on board due to only checking 1 bag in for the both of us. Next year will be fine with 2 cases.

Thank you!