I’d say don’t go over 55 liters (3900 cubic inches), and many people here will tell you can get away with less if you pack really light. I’m not a super light packer, so I like a little cushion. The absolute best way to buy a pack is to go to a store and get a pack that fits you really well. You’re in or near Portland, no? REI is one place you can try. You can go there, try on different packs and then check online to see if you can find the one(s)you like cheaper. You seem to fall within average height range, so if you found a great deal on a women’s pack online and couldn’t try it on first you’ll probably still be ok.

make sure whatever you get you talk to someone and have them fit it on you correctly, its very important for comfort and weight distribution while your walking for long periods of time.
ill be in europe in october im really excited. ill also be in milan and ideas of fun places to hang ? cool stuff to see?

The biggest mistake is packing too much, because after some time things add up and you won’t be able to carry your bag anymore.
What time of the year are you traveling? Because this will also have a major impact on what to pack!

Make sure you get a backpack with strap that go around your waist. They prevent your backpack from flopping around and helps distribute the weight. My advice is to bring neutral colored shorts and pants, that way whatever top you wear it will match. You may also want to bring a cheap elastic waist skirt that you can slip over your shorts (you’ll need to cover your legs a bit to go into many churches).

Well the main thing is as 850Mikeb say dont bring to much.

And the good thing is that no of your friends will know how you dressend in Europe.
So take the absolute minimum and plan to wash every week.

5 t-shirts, 2 shirts (protects for sun), 2 trousers, 1 shorts, 8 underwere, 8 socks, colligeshirt, worm sweter, thin rainproof jacket, gym shoes, swimmingdress, towel+soap.
And this INCLUDES what you dress in on the plane. You probably have some stuff you plan to throw away, use it last time in europe and get rid of it when you find a nice t-shirt from London. And dont bring any beauty products, you cant spend a hour in the bath in this bloody expencive tripp.