Beer keeping Romeo bigger (6)
I believe the answer is:
larger
'bigger' is the definition.
('large' can be a synonym of 'big')
'beer keeping romeo' is the wordplay.
'beer' becomes 'lager' (lager is a kind of beer).
'keeping' indicates putting letters inside.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'lager' placed around 'r' is 'LARGER'.
(Other definitions for larger that I've seen before include "Bigger, greater" , "more stout" , "Greater in size" , "Comparatively big" , "More bulky" .)