Different sizes under discussion (4)
I believe the answer is:
else
'different' is the definition.
(synonyms)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for else that I've seen before include "In addition; different" , "In addition, besides" , "Besides; different" , "Other than (what is under consideration)" , "Otherwise; different from" .)