This is what is available on the Olympic Airlines' website:
For safety reasons, a passenger can carry into the cabin only ONE piece of hand luggage, the dimensions of which shall not exceed 23 x 36 x 56 cm in total.

And for the checked in baggage the allowance would be 15kg for domestic, but usually not a problem having 20kg. with you.

Check Aegean Airlines too, they have pretty similar regulations concerning checked in and carry-on baggage.