Size of a Christmas stocking (4)
I believe the answer is:
mass
'size' is the definition.
(thesaurus)
'of a christmas stocking' is the wordplay.
'of' indicates the answer is hidden within the clue.
'MASS' can be found hidden inside 'christmas stocking'.
(Other definitions for mass that I've seen before include "congregation for > service" , "Widespread" , "Total" , "Matter; large amount" , "Heap; religious service" .)