Here are some to try:
ULA Equipment (link above)
Six Moon Designs
Hyperlite Mountain Gear
Gossamer Gear
Elemental Horizons
Mountain Laurel Designs

These all make lightweight packs with frames or with removeable stays. They also make frameless packs, but you probably don't want one of those unless your total pack weight is 20 lbs. or less. Some are comfortable with up to 25 lbs. in a frameless pack. I can't carry more than 8 or 9 lbs. without a frame. Fitting a pack is comparable to fitting shoes (everyone is different), with the additional factors that the pack has to fit your gear, fit your body, and ride comfortably on you with your gear/food/water inside. Note that the GoLite Jam is a frameless pack.

With a box with vertical sides, you can measure the approximate volume of your gear by packing in your gear plus the equivalent of food and water, then making the top surface as level as possible. You can then measure length x width x height and come out with volume. If it's in inches, 1 liter equals approximately 60 cubic inches. Approximate is good enough; many pack volumes are not all that accurate since there is no set standard.

Before ordering, be sure to check the manufacturers' websites for the recommended load you want to carry in the pack, and compare it to your total pack weight.

When ordering a pack, you need to expect to pay the shipping charges to send a few back. Have all your gear (including the weight/bulk equivalent of a week's food) ready, load up the pack as soon as it arrives and take a "hike" of an hour or two around the house.

You really should follow the same testing method when trying on a pack in the store.