70L is definitely too big. I believe the 55L is a 40L + a 15L daypack. This will probably work, but double check the length, width, and height, and make sure none of those dimensions are larger than allowed for a carry on.
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Extended-trip (5+ nights; 70 liters or larger)
Trips of 5 days or more usually call for packs of 70 liters or more. These are also usually the preferred choice for winter treks lasting more than 1 night.
Yet all being said, there is a loose one-size-fits-all approach for most travellers. The truth is that if you're looking for a versatile backpack for travelling Europe, South East Asia or even South America, a 40L backpack will (probably) be the perfect size.
If you will potentially need that extra capacity on a future trip, then definitely get the 75L pack. If you're doing it just because it doesn't seem like much extra weight, even if you don't expect to ever need it, I would go for the 65L pack. That would also curb the temptation to overpack.
The design of the 70L is perfectly suited for airplane carry-on or check in, and the features of the 70L are perfect for any adventure once you arrive at your destination. Being able to compress the 70L allows you to use it as carry-on for all major airlines in the US and Canada.
Carry-on Luggage Size MattersGenerally, most of the European airlines limit carry-on luggage to a maximum size of 55 x 40 x 20 cm ( 21.6 x 15.7 x 7.8 in); however, a few airlines do accept bags up to 25 cm wide (9.84 inc) or do not accept more than 35cm tall (13.7 in).
Size Matters (but Bigger Isn't Better)Like a few other things in life, size matters when it comes to choosing a backpack — but not in the way you might think. Based on many years of dragging excessively large backpacks around the world, I have to say bigger is not better. You do not need an 80 or 90 litre pack.
Carry-on bags are limited to 22 inches long, 14 inches wide and 9 inches high including wheels and handles. All three measurements shouldn't add up to more than 45 inches.
The one-size-fits-all approach – a 40L backpack. Yet all being said, there is a loose one-size-fits-all approach for most travellers. The truth is that if you're looking for a versatile backpack for travelling Europe, South East Asia or even South America, a 40L backpack will (probably) be the perfect size.
24-26 liters is considered a medium-size backpack and is probably the most popular bag capacity out there. It's great for taking to school or work, depending on the styling, and can also be a great travel pack for taking on a plane. Here is a potential packing list for that size depending on where you're headed.