How much actual walking will you be doing? I was in europe for 3 weeks and I used a duffle from arcteryx
Carrier Duffle 35 / Sacs-à-dos / Pour une nuit / 30L à 40L / Arc'teryx / Arc'teryx
I find backpacks are great, but they have too many straps and compartment that can get caught on the plane. Having a duffle means I have one big space. I then use smaller zip bags to separate things like underwear, dirty clothes, misc. stuff.
i would say most 30-40 L packs will be safe for international carry on. if you travel within europe, then some flights might tell you to check in a 40 (which has happened to me in the past)