90 liters is definitely too big. This is the size expeditions use when they bring 2 tents, sleeping bags, all their food and water, cooking gear and science equipment.
60+10 liters is also too big, unless you intend to travel with a tent, cooking gear, air mattress and sleeping bag.
A sensible size to bring would be a 40+10 backpack. After all, all you need is 3-4 changes of clothing. You can easily wash your clothes in hostels, you don't need to carry your whole wardrobe around with you. If you don't need an item every single day leave it at home. The goal is to carry no more than 15 kgs when you leave. If you are small and weigh less than 60kgs, 10 kgs is the max you should carry. (A fully packed 90 l rucksack weighs around 30 to 40 kgs, go figure.)
If you are a girl I can recommend the Deuter ACT lite 45+10 SL to you. Just the right size with room to spare. (I got the slightly smaller 35+10 SL version and am very happy with it.)
http://www.deuter.com/en/products/print/33748_ACT_Lite_45+10_SL_556_ENG.htm
I saw the above rucksack on ebay for 100 GBP including shipping to the UK from Germany. (See item number 180188021348 on ebay. I've bought from the seller before and am happy with them.) But you might be able to get it cheaper elsewhere.
The best backpack however is the one that fits you best. Backpacks are like shoes, you can't order them online without knowing whether they fit. Go to a good store and try backpacks on, good shops will have weights ( at least 10 kgs) to put into the backpack. Walk around with them, try to see which one is comfortable. If one fits well take that one, regardless of price. You'll be carrying your backpack a lot, it pays in the long run to have something good. Your backpack will be your friend for years, be picky.
@david h: forgiven, has happened to me too :-)