The sheer size of the master’s swollen head (4)

I believe the answer is:

mass

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'the sheer size' is the definition.
'mass' can be an answer for 'size' (thesaurus). I'm not certain of the 'the sheer' bit.

'the master's swollen head' is the wordplay.
'the master' becomes 'mas' (I can't justify this - if you can you should believe this answer much more).
'head' indicates taking the first letters.
The first letter of 'swollen' is 's'.
'mas'+'s'='MASS'

'of' acts as a link.

Can you help me to learn more?

(Other definitions for mass that I've seen before include "Rite; quantity of matter" , "what 13 15A [STANDARD MODEL] supposes 12 [HIGGS BOSON] gives other 5A [PARTICLES]s" , "state, briefly" , "Great deal" , "congregation for > service" .)

I've seen this clue in the Evening Standard.

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