Have you tried gathering all the suff you'll want to carry with you (or similar things of similar sizes if you haven't bought everything yet), and measuring how much space it takes up?
I like Karrimor bags and they do them with loads of money off in Sports Direct so they're a good price. Idk what size though, I'll let somebody else answer that Q :P
I have a 55L Vango bag that I absolutely love! I'm 5ft6 and slim (re: weedy arms!) and never had any troubles carrying it when full. It could carry a surprising amount and was a side loader so it was easy to sort through. It cost me £55 from TKMaxx and had a waterproof cover which definitely came in handy! It also had a day pack on it (20L) that detached which was really useful when hiking etc.
Best thing to do is to simply try it and see. Also remember that you're the one that's going to have to lug the bag around with you so be practical and don't load stuff you don't need.
Enjoy your trip and be safe.
You sound like you have not done this kind of thing before...
Without trying to sound too patronizing, it depends on what you take. Warm clothes take up a lot of space so time of year/area of europe is important. Sleeping bag (how big if so...)? Tent? How many normal clothes - some people don't mind rotating 2 sets whereas others seem to try to pack a new set for every day! As a general rule toiletries and 'extras' (like a camera, maps, books, diaries etc) always take up far more space than you'd thought. Lay out the stuff to see how much it is. Do you have any bags you can borrow to test how big you need?
Pack light and you might get away with 35 litres, although i'd lean on the side of caution; having to cram a bag until it is absolutely full multiple times a day is definitely NOT preferable to having a slightly bigger bag! Bear in mind that most people doing 'backpacking' will have big 60-70L bags, often because they pack too much.
Very useful advice everyone cheers. I've decided I'm not taking a tent or sleeping bag as I'll stay in hostels/sleep on trains, so hopefully I'll be okay with a 35L? One of my friends has a bag so I'll gather some stuff and see what I can fit in to that as a couple of you suggested. Thanks