I've found that deck lines any less than 6mm in diameter are tough to hang onto, so that's the size I use.
I've also added RED plastic tubing over the lines near the cockpit, where a paddler I'm assisting to re-enter can hold. It's pretty easy to direct them to grab the 'red things' and for that reason I'm starting to replace the end toggles with red-coloured handles.
Regarding the reflective traces, I haven't found cordage that is big enough so I forgo that option and just apply reflective patches on the boat. With a kayak we aren't short of surfaces to use, unlike reflective tent guy-lines for example.