Gentlemen,

I'm looking for a good backpack that can outlast me. I'm reluctant to buy from a civilian department store assuming that military grade backpacks are made to last.

The problem is I'm not too familiar on what makes a good backpack. I also would like your experienced opinion on the optimal size of a backpack.

Could you tell me which material is the best in terms of its durability?
What size backpack is suitable for 3-7 day hike?