Unless you can afford a lot of packs, probably not. Besides, a pack that's too big for your stuff will not carry correctly.

Don't even think about ordering a pack until you have all your gear, (plus, as I said, the equivalent in weight and volume of a week's food and a day's water). Weigh it all, then pack it in a box and measure the volume (length x width x height). This will give you an approximate measure (do note that pack volume is a very approximate number anyway). Since most pack sizes are now in liters, note that 1 liter is approximately 60 cubic inches. When researching packs, be sure to check pack specifications on the manufacturers' websites--subtract 5 lbs. from the recommended maximum weight since for most people that number is overstated.

Leave your stuff in the box. As soon as the pack arrives, pack your stuff in it and carry the load around the house for a couple of hours. Be sure to leave the tags on the pack and keep it clean, so you can return it if it doesn't work.

I was lucky when I ordered my pack--the first one I ordered fit as though it had been custom made for me. I was fully prepared to pay return shipping on half a dozen packs to get one that worked for me.