Most of silver parts reduced in size (9)

'size' is the definition.
(I know that largeness is a type of size)

'most of silver parts reduced' is the wordplay.
'most of' means to remove the last letter.
'silver' becomes 'argent' (synonyms).
'parts' means one lot of letters goes inside another (in the sense of cuts or separates).
'reduced' becomes 'less' (a reduced amount).
'argent' with its last letter taken away is 'argen'.
'argen' put inside 'less' is 'LARGENESS'.

'in' is the link.