Someone also mentioned the Lowepro bag finder. It may work in some cases, but when I entered what I wanted to put in the bag, it provided choices that would not work and omitted many that would. I found the Pro Runner 350 AW by looking through their site the old-fashioned way. Even that's a challenge, sometimes. Camera bags are one area where going into a brick-and-mortar shop is a great idea. Bring your gear, put it in the bags, see what fits and what doesn't. Try the loaded bag on for comfort.
Online descriptions can be misleading. . . .
I'll just add, for example, that the shorter length of the Lowepro bag fit me much better and was therefore more comfortable on than the larger Think Tank bag. If I were taller, I might have gotten the Think Tank bag. I also forgot to mention in my earlier post that while I found this bag on the Lowepro site, I actually bought it at my local Calumet store after loading it up and trying it on.
Great points from VC and Neuro.
I bought a Dakine photo backpack online. It looked perfect for what I needed. When it arrived, it was much bigger than I had anticipated, much heavier, and rigid in a way that meant it was always big, even when empty. It had some great features and I wanted to like it, but it was kind of a beast, at least for me.
I ended up picking up a Lowepro bag at REI where I could try it on, see how much room I had after putting my camera in it (since I only had my body and a lens with me), and try out a few non-photo things in it like water bottles, etc. I'm really happy with it (Photo Hatchback 16AW) and I have sent the other bag back.
As much as I like the convenience of ordering online, trying the bag on in store was key for me.