I think it depends on what use you're going to be giving it. If it's as an anchor sling I prefer to have it looped (double fisherman). If its your self rescue kit, then I'll just have it tied in a big overhand, but not looped. Same applies to lengths.
I have 7m of 7mm, tied in a loop with a pair of double overhands and then looped/twisted as per my slings.
But to be honest, never been that happy with it. Holds too much shape/twist when uncoiled and it always ends up a bit short when used on triple anchors with the big knot at the bottom, rather than self equalising (forget the correct term).
I use one of these, home made out of 8mm (I think) it's as long as doubled over fits across one shoulder and across my chest probably 3m ish from eye to eye. It's great very versatile doesn't get twisted or in the way, it quickly equalises 3 bits of gear, or two. I love it.
> "In case it is relevant, both ears are normally on one krab, so not sure what the breaking force would be "
> may as well make a loop then, whole point of the bunny ears is they can be used individually.
Not quite. Note I said normally, not always. Sometimes one will use a single arm, and untying a double fisherman's takes awhile. If using a single arm, one needs to understand the limitations and the likely forces (second rolling down a slab, vs 20m FF2). But the main reason is that with a loop the knot tends to end up in the wrong place. Finally, cordelettes end up being used for a multitude of things, and starting with a length of cord rather than a loop is often easier
The solution I see more and more frequently is tying the cordelette with an EDK, which is relatively easy to untie in case one wants to thread. In Europe they seem to be using a variation of the EDK which uses a barrel knot (1/2 of the standard double fisherman's) in place of the EDK's overhand.
Here's a professional video from Orthovox about threading a cordelette through many anchors, as happens in the Dolomites. https://www.youtube.com/watch?t=125s&v=jkx02ANJiDY They use the standard EDK there.
What are the advantages of a cordelette over a 240cm sling? As far as I know:
You can untie it to reach potentially spaced pieces more easily. Is this why they're popular in the states, as you're more likely to be climbing on a single rope?
It isn't weakened as much at the knot as a sling? (although an anchor shouldn't receive a load big enough for this to matter).
I've never understood why someone would carry them over a long/multiple sling(s). They seem quite bulky in comparison.
It depends what you want it for. If you want to equalise bolted/pegged belays, then probably circa 240cm when tied in a loop (so... 5m-ish). Or... better still, get a 240cm 8mm dyneema loop. To rack cord, fold it until it's as small as possible, then tie it in a large overhand knot with a carabiner securing the shortest end. For dyneema takes, fold them and twist them a couple of times with each fold, secured by a carabiner.
If you expect to do lots of abseiling on a long route, you might take more cord and store it in your rucksack rather than on your harness.
Pretty much the untie is the main benefit. That being said also sling snake-a-lets exist.
the other benefit is that 4 to 5mm cordelette is s lot cheaper than 240cm dyneema sling. Plus you van more easily rig rappel anchors from the cord more easily.
that being said, I mosty use a 180 dyneema sling as my ”cordelette”. 99% of time it has been enough. In fact I more often just use a 120 dyneema, as I carry those anyway as possible runners.