For example, American Airlines requires that the sum of three sides does not exceed 36 inches (92cm), and Northwest Airlines requires that it does not exceed 15 inches × 11 inches × 6 inches ( 38 cm×28 cm×15 cm). Therefore, if you bring a 60L backpack, don't pack it too full.
How many average days of clothing can you fit into each of the sizes? The answer to this varies greatly from person to person and destination to destination. However, you can expect to get at least 3-6 full days of clothing and gear in a 40 liter pack, 6-8 days in a 55 liter bag and more than 10 in a 70 liter bag.
While it may vary from airline to airline, the maximum duffel bag carry-on size is typically 22 x 14 x 9 inches for flights within the U.S. Bag size limitations on international flights can vary, so always check ahead of time on the airline's website.
The usual maximum dimensions for carry-on luggage are 9 inches x 14 inches x 22 inches (22 centimeters x 35 centimeters x 56 centimeters). This is 43.12L, so your 50L backpack is about 16% too big.
The 70L pack size has enough space for 7-10 days of food and ultralight gear. The pack can be compressed down with the roll top and side compression straps for short trips or use the top strap and base straps to lash extra stuff to the outside for long carries.