Large tome’s bulk (6)
I believe the answer is:
volume
This clue is a double definition.
'large tome's' is the first definition.
(large book)
'bulk' is the second definition.
(I know that bulk can be written as volume)
(Other definitions for volume that I've seen before include "Gross < sound" , "Amount of three-dimensional space occupied" , "Loudness of sound" , "Three-dimensional space occupied by something" , "Publication" .)