I use 600 feet of 3/8 inch polyprop (floating) line. From what I have witnessed over the years of watching others stern tie, it seems the difficult part is getting the line back to the "mothership" if you want the line to go to shore and back. To solve this, I take 2 lines ashore, the stern tie, and a much thinner "messenger line". I take both lines ashore and pass the stern tie through the ring (or rings - explain that a bit later), and tie a bowline in the end of the stern tie line. I then clip the messenger line to the bowline, and my wife on the boat reels in the messenger line bringing the stern line back to the boat quickly. On shore, I can pull slack from the mothership to ease the pulling for my "bride". We use a small electrical cord reel for storing and retrieving the messenger line. Works like a charm! Once the end reaches the boat, my wife cleats it off and pulls in what slack she can in the "bight" of the rope (first leg of the trip ashore) and cleats that off when she can't pull anymore. I then easily return to the boat and tighten up as needed.
If using a stern tie ring that is it. If going around a tree, I then use a "bridle" made out of 5/8 inch 3 strand polyprop. This bridle is about 6 feet long, with a small metal eye spliced in one end and a larger ring spliced into the other. On shore, put the bridle around the tree. Then pass the stern line through the small eye first, followed by the large eye. Attach the messenger line, and see above. The bridle eliminates any "sawing action" on the tree, protecting it.
To leave without going ashore, uncleat the free end of the stern line, and tie a knot in the end. Retrieve the line as usual. The knot will pass through the large eye, and "catch up" on the small eye, dragging the bridle back to the mothership with the stern line. I have never damaged a tree yet, and only had the system 'hang up" once necessitating going ashore to free up the snag.
While deploying (due to wind or current) we have used almost the entire 600 feet one time, so it was nice to have it.