Former shelter of some size (6)
I believe the answer is:
extent
'some size' is the definition.
(thesaurus)
'former shelter' is the wordplay.
'former' becomes 'ex'.
'shelter' becomes 'tent' (tent is a kind of shelter).
'ex'+'tent'='EXTENT'
'of' is the link.
(Other definitions for extent that I've seen before include "Area; size" , "Bulk - compass" , "reach?" , "A cake; last word of Macbeth" , "stretch" .)