I can't tell you exact lengths but can offer a word of advice.
A buddy of mine is a 15,000+ hour float pilot here in AK, he has flown for many of the guide outfits. A few years ago he was on a trip through lake clark pass and hit a little moderate turbulence in a turbine otter.. one which he has many many hours in. One gust hit him JUST right and the rope on the wing flipped up and got caught in the aileron. He was barely able to keep wings level using rudder and a bit of a crab till he got to a lake and was able to set it down and cut the ropes. These ropes of this length had been on the plane for many many years without any issues, but that one time and the set of circumstances that even Vegas would not give you odds on was enough to damn near take out 10 people in a pretty narrow mountain pass.
I have since cut mine down to a length that the rope can't flop up into the ailerons or flaps no matter what. That should still give you enough length for the dock handlers to grab them.