Large book size (5)
I believe the answer is:
folio
'large book size' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for folio that I've seen before include "outsize book" , "Four pages" , "Leaf of book - page number" , "Folded sheet - or book made from such" , "Manuscript sheet" .)