Jim,
Best advice is to buy all your other gear first. Then, take the all your gear, along with filled water bottles and fuel bottle, and food to a local outfitter. Be prepared to spend hours loading up your gear into various backpacks, and see how they fit you. Everyone is different.
That being said, no matter what I would look for a pack that is not larger than 80 liters, or 5000 ci. The reason is that the bigger a pack, the more gear you tend to carry, and that equates to weight. Very few people are happy carrying 50 pounds or more. Most like to carry considerably less.
Popular packs to consider include the Osprey Aether (as mentioned above) and the Gregory Forester.
Weight wise, the big three are your pack, sleeping bag, and tent. So, the goal is to see how small and light you can go, yet still have the comfort and safety that you need.
Peaks