You want the pack size that fits most of your needs, which is mostly your standard gear with maybe another jacket or two for winter camping should you be into that. My 40 liter pack is is good even for the winter. You should never buy for "just in case" because of what I cited earlier about your pack fitting your gear. As DTape mentioned, if you're going to hunt elk or carry a packraft, get a used external frame pack.

Your food for a long trip should be dried/concentrated (in calories as well as moisture) enough that you can get by with no more than 1.5 lbs. per day per person. There's a great site by Sarbar, one of our members, for that: Trail Cooking. Remember that on the AT there are frequent town stops where you can gorge on burgers, ice cream and fresh stuff.

Those monster packs are designed to be sold to unsuspecting beginners by unknowledgeable salespersons, along with heavy tents, heavy sleeping bags and similar gear. The purpose is to max out your credit card and enrich the store, the heavy gear manufacturers and Backpacker Magazine, and never mind if you can even manage to hoist the pack onto your back!

Read the articles on the home page here, and you'll find a better way!