There’s no one right-size backpack for travelling in Europe. Light packers are unlikely to need a backpack more than 40-50 litres. Bags on the smaller end of this range can usually be taken as carry-on for international airlines too, so you won’t need to check a bag!  For travellers wanting a few more luxuries, a 60-65-litre bag should be enough. Any bigger and the bag starts to become unwieldy – especially when navigating tight city streets or public transport!