I think you're asking an impossible question - until I can see the pack on you, loaded with gear, I can't begin to know whether it fits. Not knowing where you are located, I can only offer a solution that may or may not work for you. It depends on shipping costs.

I'd suggest you order from a company that allows you to return anything you haven't used for a full refund; I believe this is still REI's policy. Then, order the pack you want in both sizes. When they arrive, try them both on with your own gear packed inside, and make your choice. Then send the other one back. (Don't take them both out backpacking to decide; you may not be able to return one that has actually been used on a trip.) You will have to pay the shipping cost, but that's still cheaper than if you had to go to the nearest store that carries the pack.

Also, keep in mind that one possible outcome of this experiment is that neither size will fit you right, and you may end up sending them both back and looking for a different pack.