Be the right size or shape (3)
I believe the answer is:
fit
This clue is a double definition.
'be the right size' is the first definition.
(synonyms)
'shape' is the second definition.
(fitting is a kind of shaping)
(Other definitions for fit that I've seen before include "Able-bodied" , "Paroxysm" , "Of suitable quality" , "Ready" , "Apt" .)