Book or paper size (6)
I believe the answer is:
octavo
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">book</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that octavo is a type of book)</span><br/ ><br/ >'<span class="explanation-format__original">paper size</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(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" .)