Way too subjective to be a simple FAQ here. In the interest of keeping this simple, 65L is a good backpack for most of our trips (5-10 days). Anything smaller and you need to have your gear system dialed.
Is it doable to do a 10 day backpacking trip with a 45L backpack? Yes. Does it typically work well? Not at all.
For a more thorough discussion of appropriate backpack size, see this blog post.
