A prusik loop is a length of 5 or 6mm cord tied into a loop using a double fisherman’s knot. Prusik loops can be any length but using a length of rope between 120cm and 160cm will give a pretty flexible system.
The knot must be tied with a rope of diameter less than the main rope. So if you are using an 8mm or 9mm throw line you need to be thinking about a prusik loop made fromm 5 or 6mm cord. The closer the cord and main rope are in size the less efficient the system. You also need to balance this against using a cord that is so thin that a. its breaking load is quite low, and b. it could cut through the main rope.
I made a couple of new loops today. Each one takes only a few minutes to put together. The 5mm cord used in the following images is manufactured by Marmot and costs only £0.60 per metre. The first step was to cut the cord to length and then seal the ends using a lighter. I cut my cord to approximately 150cm long.