The answer is, of course, it depends. The pack has got to fit your equipment and packing style. The standard advice is to buy the pack after you get your other equipment. You can bring everything with you to the store so you can see how the pack fits you with your equipment. That's great advice. I will add its not a bad idea to get a pack a bit smaller than you think you need to prevent you from bringing all the stuff you don't need.

I use a 60 L pack which I got as a newbie. When it was new I thought it was too small. Now, even with a bear cannister, it is too big for me by myself. As luck would have it I've started taking my son backing with me and his gear has more than filled in the missing space. If I was going to go by myself more, I would probably look at 45-50 L bags.