I feel like for most beginners, 40L is the least they'll want to take, calling it too big is disingenuous I think. My wife and I both had 40L packs, the size was good, at then end perhaps we could have gone smaller, but not much below 35L each.

At the start, 40L was really tough and I think for someone doing this for the first time, 40-50L is ideal. The focus on bag size can be almost fetishistic for some long term travelers. Those who travel with two shirts, one pair of pants, and a laptop are going too far in the opposite direction in my opinion. I don't want to spend all my time doing laundry or smelling awful.