I found that the best length is one that when the sling is hanging off your hip, your feet don't get tangled in it. Too long and you end up having to daisy chain the thing, which takes up time. In trying out longer slings in the past, I found if the sling was daisy chained, I was less likely to use it. If I did use it and I'm in the middle of a job, it was unlikely I would daisy chain it back up, and then it's getting tangled in your feet.
If you need a longer sling, it's much advised to get an endless loop, polyester rated sling. They're not so expensive and they're quite bombproof. They're what you're gonna want to use for hanging a block, big rigging, footloop for spikeless crux zones and even heftier ones for crane work.
As far as your day-to-day accessory slings that you wear to do regular tree work and light-medium rigging, using tubular webbing that you'll tie up yourself, start here; Spread your arms wide, fingertips as far apart as you can get em. Cut a length of webbing that goes from fingertip to fingertip. Tie your sling from that length and see how it works for you. Call this your reference length. If it's too long and gets hung up in your feet, go shorter on the next one. I find this wingspan length of webbing makes an excellent all-around length of loop. However, that's just because my arms are as long as they are and my legs are as long as they are. This length suggestion is just a rough gauge, a reference place to start from.
