Another suggestion would be to rent a pack from your local outfitter for your first few trips. They will measure you prior to the rental to insure fit. Ask if they still sell the models you are renting. If you like that pack and it works you can buy the same type from them. If its an older pack or your budget doesn't afford the same pack new look on ebay or other used gear forums to find the same pack in the same size used. You can usually rent a pack for a weekend trip for $20-30.

If you are in the Greenville area I hignly recommend Appalachian Outfitters in Greenville SC if your in the upstate. Their staff is super helpful & they rent quality gear.