Is the right size (4)
I believe the answer is:
fits
'is the right size' is the definition.
('fit' can be a synonym of 'be the right size')
This is the entire clue.
(Other definitions for fits that I've seen before include "crises once" , "Paroxysms" , "Seizures" , "short bouts" , "Sudden uncontrollable attacks" .)