That's really difficult to do as it really depends on what you want to lift. I seem to use the shorter ones more often. I have a 6-footer that just reaches between my two bucket hooks with a little droop. I use it for carrying my 250lb generator back and forth to the garage. I can simply run the sling through the crane loop on the generator and then attach to each bucket hook. The same length works well for carrying my mower deck. I would say that like chains, the various lengths don't really have specific applications.
I think I have a couple 4, 6 and 8-footers which so far have served every purpose I've ever needed.
I would start out with a couple 6 footers and see how they work out for you. If you go with longer ones you can always double them up for shorter uses.