High THC!
Here's a pic of where I cut my clones....As you can tell there's probably a 101 ways to get a clone to root from all of these different takes on the subject so there's no way I can tell you this is the absolute fool proof way but it works great for me...
I like big clones because they are usually a better specimen than something small. Once they do take root I only need to veg for maybe a week and they're off to bloom.
With small clones they take longer to root and longer to get established as small plants once they do take root.
The cutting above is huge and has about two more nodes than what I usually take but when I have the opportunity to take a biggin' like this I jump at it.
From the very top shoot I count down at least two nodes. That part of the clone from the second node and up will be above the neoprene sleeve. Below the neoprene is an additional two sets of nodes. The leaves at the two bottom nodes are removed to create additional rooting points.
I dont really go by length of the cutting but number of nodes dictate to me how big the clone will be.
Some growers score or slice an additonal mark on the stem but I dont. I always see roots come out on virtually every part of the stem, the cut node, the bottom cut, and every other place up and down the stem will have roots pop out.