Go to a good local hiking/climbing gear store if you can find one, or to the nearest REI or EMS, get properly fitted and try on a lot of different packs with 25-30 lbs. weight in them. If nothing else, this will teach you how to fit a pack, what size you need and just how a comfortable pack will feel. Don't order by mail/internet until you have done this. If you don't buy a pack at the store (and you may want to consider your purchase before doing so), then buy a little something to help reimburse the store for their time. (At least your conscience will feel better.)
One problem with a pack that's bigger than what you need is that it does tempt you to take a bunch of extra stuff, which adds to the pack weight. Also, for most packs, it's hard to have a pack load properly balanced when it's under-filled, although a good network of compression straps can remedy this problem.
Go to the home page of this website (see "TLB Home" to the left) for articles on shopping for a pack (lefthand column) and many other important backpacking advice.
While REI is liable to try to sell you a 6-7 lb. pack (don't succumb!), their info on pack fitting on their website is excellent:
Finding your torso and hip size Adjusting the pack Assuming you aren't ready to go lightweight at this point, look at Osprey packs. They are pricey but, for many people, extremely comfortable. I almost bought one myself, but wanted a lighter pack. (Osprey has come out with a couple of lighter models this year, under 3 lbs.--if they'd had them three years ago, I'd have bought one.) My son has an Osprey Aether 85--he's a surfer so carries his wet suit (very heavy when wet) plus acting as a Sherpa for his kids, so he needs a large, very supportive pack. However, pack fit is almost as individual as shoe fit--get what works for you.