Limit an urban area's dimensions (8)
I believe the answer is:
capacity
'dimensions' is the definition.
(thesaurus)
'limit an urban area's' is the wordplay.
'limit' becomes 'cap' (capping is a kind of limiting).
'an' becomes 'a'.
'urban area' becomes 'city' (city is a type of urban area).
'cap'+'a'+'city'='CAPACITY'
(Other definitions for capacity that I've seen before include "Size of stadium" , "how much can one take?" , "Space -- role" , "The amount that can be contained, space" , "Potential - role" .)