Book size (6)
'book size' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for octavo that I've seen before include "An eight-leaved book or page" , "Size of book page" , "Common book size" , "Size of a book, pages are made by folding a sheet of paper three times to form eight leaves (6)" .)
(Other definitions for volume that I've seen before include "Aggregate" , "Amount of three-dimensional space occupied" , "Space measured in cubic units" , "Space occupied by an object" , "The bulk" .)
'book size' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for folio that I've seen before include "Body of written work" , "File" , "Four pages" , "Leaf of paper" , "Sheet of paper folded to make two book leaves" .)