Buy your gear and then your pack
The comment above about it depending on how lightweight your gear is was a critical one. Don't worry about what size pack to get until you get what will go into it. One person may be able to fit his standard gear (sleeping bag and pad, tent, cooking gear, clothes, water filter, etc.) and the food to be taken into a 5000 cubic inch bag while someone taller or or with bulkier gear may need a 6500 or 7500 cubic inch pack for the same list of equipment.
Once you have your core set of equipment lay it out with something the same size as the food you will be taking into a rectangular cube and then measure it to see how many cubic inches you will need. Also, keep in mind that most backpack manufacturers measure the capacity of their packs based on a fully stuffed calculation which takes into account stretching. This means that a bag that is 10" x 10" x 20", or 2000 cubic inches might be rated as 2200 because you can jam 2200 ci's of stuff into it, so get something that is 10-20% larger that what your rectangular cube measures. This will also make it easier to pack for the trip out and you are sitting on ground stuffing stuff into your pack bag rather than carefully putting it in just the right place.
Also, many packs have expansion collars on the top that they use to get their full volume, so the main pack bag may be 3000 ci's but with the expansion collar it goes up to 5000 ci's...but it is also 12 inches taller which makes it go over your head. If you are normal sized you may not like this design and might prefer a pack that is wider than some others.
Hope this helps,