does anyone have any idea how big the lockers generally are in the hostels of Europe?

Question

What can they fit? Whats the luggage storage situation?

Answer 1

Hostels have different sizes of lockers, mostly my moderate backpack had ample space, a big pack would have fitted, but I have been in a few where my pack did not fit in the locker and I had to divide my things and put the valuables in the locker, leave the rest in the room.
Not all hostels have a locker in each room, in that case, if you can lock your pack closed and to your bed, you can leave it out. (Often I would not bother to lock my things in a hostel like that, I would just keep my camera and passport and money with me.)

Bring your own pad-lock, mostly you can buy or rent them at the reception desk, but it is easier if you carry a good one you trust.

If you travel hostels it is a good thing to travel light, as you will need to carry your things more than you would expect, up stairs, in and out of trains and lifting the pack up into the overhead rack in the train.
(That is if you travel by train, of course.)
You will find that you need less clothes than you would need for the same time at home, and doing laundry underway is never a problem.