Nothing under two pints of size for paper (6)
I believe the answer is:
quarto
'size for paper' is the definition.
'quarto' can be an answer for 'size' (I have seen 'Size of book' mean 'quarto' so perhaps 'size' could also mean 'quarto'). I am unsure of the 'for paper' bit.
'nothing under two pints' is the wordplay.
'nothing' becomes 'o' (O resembles 0).
'under' says to put letters next to each other.
'two pints' becomes 'quart' (unit of liquid equal to two pints).
'o' put after 'quart' is 'QUARTO'.
'of' is the link.
Can you help me to learn more?
(Other definitions for quarto that I've seen before include "Book page size" , "Ten-by-eight inch paper" , "Publishing term, four leaves are formed from one sheet" , "Twice-folded paper size" , "Book size resulting in folding one sheet four times" .)