Europe Travel Backpacking?

I plan on backpacking europe for 2 months starting in sep. I was wondering what size backpack to buy. I have been lookin at 4000 cu. in. (60-65L) packs. I am really liking the Osprey Atmos 65. Anyone have any tips or comments on what I should look for or how big a pack I need? Any tips on bacpacking are more than appreciated. Thanks

Buy a Small one.
You will find that your pack is getting heavy very fast.
If you travel for 2 months you will need to do laundry in any case, so better take a smaller pack and wash a little more often.

Some packs come with a smal pack that you can take of and use for a day pack, resist them.
The heavy things like books go in them, so once started on the trip they never go back on the big pack. Because the weight will be in a bad spot, pulling the whole pack down.

Put the books in your 'big' pack, and use your day-pack for lunch, important paperwork and at most one guidebook.

I think you should take a pack that allows space for 3 jeans, 2 sweaters, T-shirts, underwear and socks for a week (two at most.)
You will find that most people wear their clothing a little longer while traveling, so this is enough for about 3 weeks ;o)

Unless you go camping you do not need the bigger size pack.

I think the Osprey Atmos 35 is a better choice, size wise.

Don't pack too much. If there's REALLY something you realise you need when you get there, I'm sure you'd be able to find some, like new socks or undies and stuff. You have to take slightly thicker clothes because it starts to get cold in those months, but be reasonable. Leave out hairdryers and electric shavers, just buy disposable razors and learn to live with flatter hair. You'll be dying to throw your pack in the nearest river pretty soon if it's too heavy.