I find that for general service like tying up tarps, making bundles etc, a double arm span is about right. Run the rope between your outstreached hands, twice. About 12' or 4m.

For longer pieces, just start winding it up until you get a package that is the size you want to carry. This seems to be the limiting factor for me. No matter how long it is, it will be sometimes too long and other times to short.

Remember that tying a knot to join two ropes will lower the working load by 50%. Depends on the knots of coures, but that is a good working estimate.