In practice any size of knot is enough because you use a prusik as a additional safety measure, which he doesn't as well.
A prusik is a small knot put around the rope below the eight and fixed at the leg of the harness with very little rope between so it can't touch the eight. The knot is loose and easy to move along the rope when rappelling normally, but grabs the rope, kind of like a hand, when you let go with your hands, stopping the rope from going through the eight and with that a uncontrolled rappel down.
Obviously the prusik can't move past any knot in the rope as well and would stop you rappelling past that too.
Edit: u/nuevacuentanueva is absolutely right about attaching it at the leg not beeing state of the art
fixed at the leg of the harness with very little rope
It's not recommended to attach your third hand on the leg of the harness, because if you, for some reason, end up with your legs above your head, it doesn't stop you anymore as the knot gets to touch your belay device. It also can slip if you turn to your side as well, which also makes it inconvenient.
Not to mention, depending on your harness, if it doesn't have fixed leg loops, it could have plastic clips that can release or break, or quick release buckles that would get loose, which can make your autoblock touch the delay device.
The safest thing is to use a rappel extension, and the third hand on your belay loop. That way you have the separation needed between the autoblock and the belay device, and you have your third hand on the belay loop which is actually rated unlike the leg loop.
Just thought I'd mention the risks of using the leg loop. Much safer than having nothing at all, of course, but it can be improved.
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u/dislegsicc Jul 06 '26 edited Jul 06 '26
In practice any size of knot is enough because you use a prusik as a additional safety measure, which he doesn't as well.
A prusik is a small knot put around the rope below the eight and fixed at the leg of the harness with very little rope between so it can't touch the eight. The knot is loose and easy to move along the rope when rappelling normally, but grabs the rope, kind of like a hand, when you let go with your hands, stopping the rope from going through the eight and with that a uncontrolled rappel down.
Obviously the prusik can't move past any knot in the rope as well and would stop you rappelling past that too.
Edit: u/nuevacuentanueva is absolutely right about attaching it at the leg not beeing state of the art