Your backpack is going to determine what and how much you are going to carry. You can get a small pack that weighs very little or your can get a large pack that will carry more than you physcially can.
In past years the most common pack found on the back of AT thru hikers has been the Granite Gear Vapor Trail. The reason for this is the packs reasonable price, light weight construction, and the simplicity of the design. Its disadvantage is its lack of compartmentalization and low weight rating. This shows that just because a bag is popular, does not mean it is the best pack for everyone. Ideally, look for a bag between 3000 to 4500 cubic inches that weighes less than 6 pounds. You can go heavier and bigger, its your preference. That is the range most hikers will be carrying.
Once you find a bag you like, take the time to go to the manufacturers website and look at their fitting chart. These charts are updated on a regular basis and will get you the most current information on what size will fit you best. Once your bag arrives, put some weight in and start breaking in your pack. Just like a pair of shoes, it will need to adjust to your body for the best possible fit.