Nut size (A.K.A. diameter) is the size screw/bolt that the nut is used with. The same is true of the thread count. Thus a 3/8"-16 nut fits a 3/8"-16 bolt.
Important Note: Due to the plating thickness, only hot dipped galvanized bolts should be used on hot dipped galvanized nuts.
Note: Working load limit applies to the eye nut only. The load properties of the product that it is being mated with should also be taken into account.