It completely depends on how much stuff you're planning on taking and bringing back. Keep in mind that by month 6 you might be really sick of carrying around the souvenirs from month 1. Mailing home items that you buy is always an option, if not necessarily cheap. I would argue with Michael about the size. I backpacked for 8 months last year with a 65 L pack and never had a problem, though I had to think about how and what I packed. I was doing a lot of heavy and overnight hiking, so I was carrying a sleeping bag, sleeping mat, boots, a set of clothes for hiking, and a lot of other little things I wouldn't have been if I had been train/bus travelling between fairly urban locations. I was pretty lean on how much city clothes I was carrying, but I managed. All I can suggest is to figure out how much stuff to want to take, bring home a few packs, and try to pack them. That will really tell you what size of pack you need, as well as what features you need.

If you're going to go for 6 months you'll might need something at least 5000cu in. That way you can put all your gear and week's worth of clothes in there and have some room left over for extra goods. This pack may be fine (it's a good product made by a good company), but you really need to think about the space needs you have.For a couple different options, check out packs like the Osprey Argon 110 or even some JanSport varietals (e.g. Klamath 85, Big Bear 82, etc.).Just shop around a little. If you decide this one is what you need, then that's awesome. But be sure you exhaust all the possibilities before finalizing. The worst decision you could make would be to by something quickly, get to Europe with it, and realize it's not quite what you wanted.