Size and state of Boston (4)
I believe the answer is:
mass
This clue is a double definition.
'size' is the first definition.
(thesaurus)
'state of boston' is the second definition.
(abbreviation for Massachusetts)
'and' is the link.
(Other definitions for mass that I've seen before include "state, in short" , "13 [BULK]" , "Large number of people" , "Religious ceremony" , "Lump of material" .)