In terms of kilos, a full 55 litre backpack will weigh around 14kgs approximately whereas a full 70 litre backpack will weigh around 20kg. That's too much to travel around the world with. Naturally, as a backpacker, you want to travel as light as possible so I would recommend taking a 55litre.
I recommend a backpack around 40L-50L. Personally, I wouldn't go over 65L, but some people like a bigger bag. You can always go smaller, but I wouldn't recommend anything smaller than 35L — unless you're into minimalist travel.
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.
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.