Ship's size or carrying capacity (7)
I believe the answer is:
tonnage
'ship's size or carrying capacity' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tonnage that I've seen before include "Weight of cargo - ship's carrying capacity" , "Charge for carrying" , "Capacity of ship" , "Ship's size or carrying capacity" , "Measure of cargo space" .)