Maine, a certain size (7)
I believe the answer is:
measure
'size' is the definition.
(thesaurus)
'maine a certain' is the wordplay.
'maine' becomes 'me'.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'me'+'a'+'sure'='MEASURE'
(Other definitions for measure that I've seen before include "Extent" , "Gauge" , "See 6" , "amount of drink" , "drink in an American bar" .)