The sweet spot lies somewhere in the middle: a backpack between 40-70L is fine for a long-term trip – the trick is not to stuff it full.
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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.
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.
The hip belt of a well-fitting pack will be centered on top of your iliac crest. It won't be any higher or lower. If the hip belt slides below your iliac crest while you are trying on the pack, then it is too big. If it rests on your lower back, then it is too small.
What happens if your backpack is too big? An overweight backpack causes the body to compensate for the extra weight. If you lean forward to compensate, it reduces your balance making it easier to fall. The extra weight can distort the natural curve of the middle and lower back, causing muscle strain.
While personal item size restrictions vary across airlines, most airlines consider a small backpack, purse, briefcase, or laptop bag to be a personal item. These must always fit under the seat in front of you. Many airlines restrict passengers to one personal item and one carry on bag.
Most airlines allow bags up to 45 linear inches (length + width + depth) or 22? long, 14? wide, and 9? deep. To prevent problems at the airport, your bag should be no larger than these dimensions.
In most cases, the compartment size may range from about 35 to 45 liters. In some cases, the bag may also have an expandable pull string storage for more space.