Yes, most backpacks contain about 60-75 litres. My backpack is 65lt, but has an extension that gives me an extra 10lt, so I don't have to fill it if I don't need to...I suggest, if you want to buy a big backpack (unless you are able to fit all your stuff in a day pack!), you buy one that's about 65lt, and that has an extension at the top, so if you need extra space when you are travelling, it's right there for you.
Go shopping, try all of them on, some will fit better on you than others...TP says you need to buy one that has a day pack attached to it, like a two-in-one pack...But I don't really agree with him (sorry TP!)...I love simplicity when it comes to backpacks (and believe me, I have 4 of them and I know what I like in a backpack ahahha!), because the more complicated it is, the more things there are to break/loose on it, and the weight will get unbalanced more easily (which is not comfortable....)!
And I know I've said this before, but before you take off, try your backpack...Pack all the stuff you want to bring, then take the bus with it, have a coffee and come back home...You'll get the feel of it, and most importantly, you'll know if it's too heavy! More than 30-35 pounds is too much...When your pack is full, you have to be able to lift it 3-4 times above your head...If you can't, it's too heavy and you'll hate it at the end of your travels! ahahah!
Good luck shopping and WELCOME
to the boards!