Size better with a large town (8)
I believe the answer is:
capacity
'size' is the definition.
(I know that capacity is a type of physical quantity)
'better with a large town' is the wordplay.
'better' becomes 'cap' (I've seen this before).
'with' means one lot of letters go next to another.
'large town' becomes 'city' (I've seen this before).
'cap'+'a'+'city'='CAPACITY'
(Other definitions for capacity that I've seen before include "The amount that something can hold" , "Ability, volume" , "The wherewithal" , "Maximum amount that can be contained" , "Specified role or position" .)