In terms of kilos, a full 55 litre backpack will weigh around 14kgs approximately whereas a full 70 litre backpack will weigh around 20kg.
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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. (Larger packs can more comfortably accommodate extra clothing, a warmer sleeping bag and a 4-season tent, which typically includes extra poles.)
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.
Each bag should weigh less than 23KG/50LBS. This is an international regulation set for the health and safety of airport workers who have to lift hundreds of bags daily.
When determining your pack weight, follow these very general guidelines: A loaded backpacking pack should not weigh more than about 20 percent of your body weight. (If you weigh 150 pounds, your pack should not exceed 30 pounds for backpacking.)
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.
Each passenger that pays for a checked baggage can bring a maximum of 23 kg with dimensions up to 275 cm (length + width + height). You can buy extra weight in 3kgs units, up to a maximum of 32kg per bag.