
The guidelines for carry-on luggage in EU countries specify that the luggage cannot be more than 10kg or bigger than 55cm x 40cm x 20cm.
Propably you can go with a 45L pack for your trip, but it is still too big for a carry-on. Also you may find it difficult to make it stay under 10kg limit for a carry-on. I would just check the backpack if I were you, and buy a pack cover to protect it too.
For a trip like this, you may an 45L internal frame. Externals are much more bulky and uncomfortable.