That of a packet of paste? (4)
I believe the answer is:
size
'paste?' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for size that I've seen before include "Bigness; a sealant" , "gelatinous mixture" , "Physical magnitude" , "Extent; a sealant" , "Dimensions, magnitude" .)