Limited in size, it fills well (6)
I believe the answer is:
finite
'limited in size' is the definition.
(I've seen this before)
'it fills well' is the wordplay.
'fills' indicates putting letters inside.
'well' becomes 'fine' ('fine' can be similar in meaning to 'well'**).
'it' put inside 'fine' is 'FINITE'.
(Other definitions for finite that I've seen before include "Having set limits" , "Having boundaries or limits" , "Bounded in time" , "Circumscribed" , "Having a limit, not completely open-ended" .)