I have searched out the advice of others here for the information you are looking for. I can tell you what the consensus seems to be and that I largely followed their example. There are other ways to do it but this works for me.

I started out w masons twine but ran into some failure issues. If the water in your wicking reservoir runs out or even low, the wicking action can stop. This is a likely failure when using masons twine and is largely avoidable.

I made the change from masons twine to acrylic yarn. Im not a yarn guy so this is not a technical description but I ended up w some fairly light two strand and three strand acrylic yarn. I use the two strand for everything but my larger plants which get the three strand.

I don't think length really matters. Seems like I usually end up w three or four inches of string floating around in the water reservoir.

I'm acrylic yarn all the way..but I do have an entire room full of it lol.

Yarn comes in different weight sizes from very fine 1 to a super bulky 5.

I use 4ply 4 weight untwist it and have used 1 strand for leaves and babies..2 for my 4inch pots..and currently have my 6 inch pots on two 4 ply ..I've had to go down in the amount of plies and have been upping my perlite to my mix..gotta love a damp house!

As far as where the wick should end...when I was potting mine up I tend to use a decent amount to make sure I had enough to work with and not fidgit with such a small strand. I ended up leaving the obnoxiously long length..but have trimmed them down as I got tired of them getting tangled or dealing with very long wet wicks. Mine are roughly the same length as Jeff mentioned above

I think the big thing you learn is trial and error..what works for me may not work for you etc..and with wicking you'll adjust based on your environment..plus plants neediness (some are just thirsty).

I bought Mason twine b/c it was convenient and I was ready to get on with my project.

would think this polyester will work, may be too thick - so you can use 2 ply instead of the whole rope. To check if it passes the water - drop the length of it in a glass of water - and hang the end of it outside of the glass. If it drips a bit - fine, it is not - won't work, drips too much - reduce the ply count. Some materials the ropes/twine are made repel water.

Length wise - approximately the height of the pot - plus whatever you need hanging to reach the bottom of your reservoir. Since mine are on mats - 1-2 inches outside of the pot.

The best is - you try it on a couple of pots - and decide - if it is too wet or too dry - and you can add or reduce the ply count.