Sizes and rope type have been discussed, generally lightweight polyprop line of a braided variety works great. I carry 600 feet.
You will definitely need more length than you first expect, and again, more is better, as it could prove problematic if you have a line that is 10 feet short
To protect trees, and still work well, I use a 'bridle". The bridle is made up of 5/8 inch 3 strand polyprop (about 6 feet long) with an eye spliced in each end using metal or plastic eyes.
One eye is much larger than the other. Also when going ashore, I take 2 lines. One is the shore tie line and one is basically a strong "string" use to retrieve the end of the shore tie from the main boat.
Procedure:
On shore, place the bridle around the tree, rock, whatever will be your 'shore anchor'. Pass the shore tie line through the "eyes"
going through the small eye first (important point). After the shore tie line is through both eyes, tie the retrieval line to the shore tie. Person on the boat can now 'pull in' the shore tie, while the person on shore can pull line from the boat to "lighten the load" (reducing friction) for the person on the boat. In this way, the shore tie is back to the boat in about 60 seconds or less. No muss no fuss. The end of the shore tie is cleated off, and then the "boat end" of shore line is pulled in to "tighten it up" or remove slack to the desired amount.
To retrieve without going ashore:
Tie a bowline knot (or other large knot) in the end of the shore tie line after uncleating. Pull in the shore tie. The knot will pass through the large "eye" in the bridle, but will "hang up" on the small eye "capturing" the bridle and the whole set up will come back to the boat! Once in a while, if the shoreline is very rocky, things may hang up. With the right technique (basically "jerking" the line when pulling in) you can avoid this from happening.
Using this system, you avoid chafe on your shore tie as it moves inside the "eyes" and the bridle does not move on the shore anchor (tree) eliminating chafe for both your bridle and the tree.