The only way to know for sure is to try the pack on. You can do this with on-line purchases by ordering both sizes and returning the one that is too small/large. Of course if you have a store handy with both sizes, the problem is much less. Check to be sure the merchant has a return policy if you are buying on-line. When you try the pack on, it should contain your gear plus an amount of extra weight corresponding to your average food/water load.

Don't be afraid to try on a lot of packs. You may wind up spending money on return postage but it is better than spending even more money on an unsuitable pack.

Don't buy a pack based solely on the features listed in the sales description; these can often be sewn on later if deemed necessary. Buy the pack for fit and comfort and get one a little bit bigger than you will ordinarily need. That way you won't need to stuff the pack if you take a longer than normal trip. Beware of features that you won't use. A pack with all of the bells and whistles is not much good if it doesn't fit you comfortably. Ice axe loops, for example, add weight and are not much use if you never use an ice axe.

I would put a lot of time and effort into researching your pack purchase. There are dozens of pack manufacturers and you should check as many as possible. Otherwise, you risk spending a lot of money on something that won't work. Again, trying packs on is really the only way to be sure one of them will work for you. If there is not a good outdoor store near you, a trip to a larger city that has one would not be a bad investment in time or money.