Size of old temporary housing (6)
I believe the answer is:
extent
'size' is the definition.
(I've seen this before)
'old temporary housing' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'temporary housing' becomes 'tent'.
'ex'+'tent'='EXTENT'
'of' acts as a link.
(Other definitions for extent that I've seen before include "A cake; last word of Macbeth" , "Bulk - compass" , "Magnitude - scope" , "Degree or spread" , "Area something covers" .)