There are some excellent articles on gear selection on on the home page of this site, left hand column. Also, look at the "sticky" article by phat at the beginning of this Backcountry Beginners section. He lives in Edmonton, Canada which is a bit closer to your climate than the gear lists on this site's home page.

The most important aspects of a pack are fit, fit and fit. The pack must fit your body, it must fit your gear and it must be comfortable for you with your gear in it (which will ride a lot differently from the weights and sandbags in the store). That's why it's recommended to acquire all your gear first and the pack last. Box up the gear and take it to the store with you to try on packs.

You won't take anything more for a long trip than for a short trip except more food and fuel. The basic gear is the same. By including the equivalent of weight and bulk of a week's food with your gear when you try on the pack, you'll be allowing enough room. I bought/prepared food for a two day menu, weighed each day's food (to get an average per day) and then used crumpled paper and weights to approximate 8 days of food.

There are two problems with a too-big pack: First, it tempts you to fill it up with stuff you don't need. Second, and more important, the load will shift around in your pack, unless it has really good compression straps. That will throw you off balance and will become uncomfortable after several hours.