What is the diameter of the smallest end, i.e. what size hole can it fit through as in a kayak through the scupper hole and the binder hole on a picku

Your question is the very reason online sellers often provide liberal return policies. Buy it, try it, use or return it.

!st question requires info from you:: What is the ID of your scupper? Your 2nd question: "It Depends". Since the concept of cable over chain or a steel rod is its' flexibility. So, my answer is: How strong are your fingers to squeeze the loop thru your scupper. Fingers too week, use plyers

The diameter of the cable is 3/8in (10mm).