Obviously your backpack is one of the more important pieces of gear you need. The most important thing about your backpack is that all the weight of the pack should rest on your hips. You should not feel weight on your shoulders, if you are then you are going to be hurting. If you are looking to buy a backpack you should go to an outfitter and try the pack on. They will load the pack with weight and you can walk around the store with it. The person selling you the pack should also take the time to fit it to you. They will help adjust everything and show you how to adjust it yourself. If you are borrowing a pack from someone you will want to take time before the trip to make all the adjustments to make it comfortable. If you don't do it at the start you will develop a sore spot at the beginning of the trip. Even if you make the adjustments later that spot will still continue to be sore until it has had time to heal. If you are borrowing a pack I would also recommend noting what the settings were before you changed them, so you can switch them back. It will take some time to full adjust a pack, it took me years to get my pack the way I want it to feel and I still have to adjust things as my body continues to change and my bag becomes more broken in.
You don't always want the biggest backpack available. A bigger bag is heavier (and your goal with backpacking is to keep everything as light as possible). Also, if you have a bigger bag you will tend to carry more things with you. If you go with a smaller bag then that will force you to leave some things behind, and your pack will be lighter.