Ship's size or carrying capacity (7)
I believe the answer is:
tonnage
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ship's size or carrying capacity</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)