Old court bible, originally only this size? (6)
I believe the answer is:
octavo
'this size?' is the definition.
(octavo is a kind of size)
'old court bible originally only' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'court' becomes 'ct' (abbreviation used in road names).
'bible' becomes 'AV' (Authorised Version of the Bible).
'originally' says to take the initial letters.
The initial letter of 'only' is 'o'.
'o'+'ct'+'av'+'o'='OCTAVO'
(Other definitions for octavo that I've seen before include "An eight-leaved book or page" , "Common book size" , "Size of book page" , "Size of a book, pages are made by folding a sheet of paper three times to form eight leaves (6)" .)