Very large or small - something funny about that (6)
I believe the answer is:
cosmic
'very large' is the definition.
'small something funny about that' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'something funny' becomes 'comic'.
'about that' indicates putting letters inside.
's' going within 'comic' is 'COSMIC'.
'or' is the link.
(Other definitions for cosmic that I've seen before include "Of the universe, immense" , "Great" , "Johann Sebastian --" , "Enormous" , "Coming from outer space" .)