<1 TB. I try to keep the entirety of my important files in under 1tb and so far, I have succeeded. Somehow I've managed to do this even with my photo/video iCloud Photo library. I am not counting large hoards like my ISO and movie collections.

I'm not really sure of exact sizes, but books and PDFs don't take up much space at all. I'd guess a few hundred megabytes? Wikipedia is ~20gb, IIRC.

Physically, I have paper copies of books stashed all over the place. It consumes a decent amount of space all together (~ 4 feet by 9 feet ~18 inches in volume)