Book or paper size (6)
I believe the answer is:
octavo
I believe this clue is a double definition.
'book' is the first definition.
(I know that octavo is a type of book)
'paper size' is the second definition.
(I've seen this in another clue)
(Other definitions for octavo that I've seen before include "Size of a book, pages are made by folding a sheet of paper three times to form eight leaves (6)" , "An eight-leaved book or page" , "Common book size" , "Size of book page" .)