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.
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.)
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.
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.
10 to 19 liters: For light loads and 13-inch laptops. 20 to 29 liters: Best for students and 15- to 16-inch laptops. 30 to 39 liters: For super-size workloads and weekend trips. 50 to 80 liters: For hikers and campers.
LARGE BACKPACKS (> 40L)They make for great travel bags when you don't want to check in luggage. Even if you're a heavy packer, you should be able to take a 45L pack away for a weekend. A light packer could easily do with a bag like this for a week.
All bags from 30L-45L meet TSA carry-on regulations for all commercial flights. The 60L duffel meets TSA carry-on regulations for most commercial flights.
60L Foldable Duffel Bag: This duffel bag size: 23.62 x 12.99 x 11.81 inches, folded size:10.63 x 2.76 x 8.66 inches, only 1.43 lbs, easy to store. 5 separate inside and outside pockets, 1 shoe compartment, 1 back sleeve, 60 liters capacity is enough to meet 3-4 days of travel.
60L Foldable Duffel Bag: This duffel bag size: 23.62 x 12.99 x 11.81 inches, folded size:10.63 x 2.76 x 8.66 inches, only 1.43 lbs, easy to store. 5 separate inside and outside pockets, 1 shoe compartment, 1 back sleeve, 60 liters capacity is enough to meet 3-4 days of travel.
Most beginners will carry around 30 to 35 pounds for their first few trips (including the pack itself). As you gain more experience and upgrade gear, your weight will likely decrease to around 30 to 32 pounds. Some backpackers stay around the 30 pound mark while others strive to get their weight as low as possible.
No matter how well-designed the backpack, less weight is always better. Use the bathroom scale to check that a pack isn't over 10% to 20% of your body weight (for example, if you weigh 120 pounds you shouldn't carry a backpack that weighs more than 12 to 24 pounds).