It’s a big book in my opinion (4)
I believe the answer is:
tome
I believe this is a double definition.
'it's a big book' is the first definition.
(term for a large or heavy book)
'in my opinion' is the second definition.
(to me)
(Other definitions for tome that I've seen before include "Large and scholarly book" , "Large heavy book" , "War and Peace, say" , "dictionary?" , "Hefty volume" .)