Once again, the term "backpacking" is a source of confusion. In the US it means hiking in the wilderness on a trail (or, for the intrepid, cross-country) with all your camping gear on your back. The rest of the world calls this "trekking" and uses the term "backpacking" to mean budget travel using a backpack instead of a suitcase. While this forum is devoted to the first kind of backpacking, nearly all of us have done both kinds.

Based on my own experience, I have two suggestions for you as a traveler: First, consider what you want to put in the pack. Second, consider if you really want to carry all that weight around with you! It is generally advised to get the rest of your gear first and then get a backpack to fit. However, that may tempt you to take far too much. One thing I always did before a long trip was to pack everything I thought I needed and then go for a 2-3 hour walk with the pack on sidewalks and up and down lots of stairs, wearing the same footwear that I planned to wear for the trip. It was amazing how many items became non-essential by the time I got home! I found that one change of clothes (except socks and underwear, of which I took three changes) was plenty if my clothes were all quick-drying. Spot-cleaning of outer clothing and nightly rinsing of socks and underwear in the hostel sink kept me going for 2-3 weeks between stops at a laundry. I don't know just how big my pack was, but it was a convertible pack (with backstays and a padded hipbelt) that met (and still does) the carry-on requirements of US airlines.

The best source I've seen to answer the conundrum of space vs. weight for travelers is this sticky post on Lonely Planet's Thorn Tree. It has a link to an excellent web page on the various sizes of backpacks (the travel kind) and what you can put into them. I hope it will answer your question!

Have a great trip!