For people with smaller frames/lighter weight the 10ft should work just fine especially for rails and smaller features, but like a lot of other posters have said on this thread, the 20ft is more versatile. It can do everything the 10 can, by adjusting the length of the lead line or reducing the length you pull it back.
wiscoboy96 is right though about needing more help to stretch the 20ft.
Also, from everything we have seen, people who try to make their own are not only putting themselves at a higher risk of it breaking and all that, but usually end up wasting time and money on something that doesn't do the trick.