Volume finished with old size of paper (6)
I believe the answer is:
quarto
'size of paper' is the definition.
(I've seen this before)
'volume finished with old' is the wordplay.
'volume' becomes 'quart' (quart is a volume of liquid).
'finished with' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'quart'+'o'='QUARTO'
(Other definitions for quarto that I've seen before include "Publishing term, four leaves are formed from one sheet" , "Book size resulting in folding one sheet four times" , "Twice-folded paper size" , "Ten-by-eight inch paper" , "Book page size" .)