The best advice has always been: get all your stuff together first, then get a pack it all fits into. You can tell if it all fits because you put it into the pack, either at the store or at home (and you return the pack unused, if it is too small to hold your stuff).

If you are going to carry all your food from the get-go, be sure to leave room for it, too. Two weeks' worth can take up a fair bit of space. Also, be sure to wear your loaded pack for a while, to figure out if it has issues around fit, such as chafing or sagging onto your shoulders. Pack fitting can be as individual as shoes.