Size of former shelter (6)
I believe the answer is:
extent
'size' is the definition.
(I know that extent is a type of size)
'former shelter' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'shelter' becomes 'tent' (canvas shelter).
'ex'+'tent'='EXTENT'
'of' is the link.
(Other definitions for extent that I've seen before include "Scope, area" , "Area; size" , "Measure" , "Length" , "Area something covers" .)