Considering these both try to wick so that you've got about a .50 mm hole in the top section of the coil open.
Here let me better illustrate with a quick text drawing: (-)
Looking at it on the side of an open coil you'll see what seems an open tube through the coil atop the wick material. The wick material will just lay on the bottom portion of the coil. I often find this helps avoid dry hits, burnt tastes. It also improves your airflow overall.
As far as wick length, just enough to brush over the bottom of your deck's juice wells. Any more than that is wasted and leads to blocking airflow. Rayon holds juice a little better than cotton and won't dry out as quickly on you. Though you don't want to dry hit rayon.
Muji cotton I find is about the best cotton to use. You can get it on Fast Tech, apologies for not linking to it. I don't do any promotional type of stuff, I'm just too busy at the moment to link. *chuckles* It doesn't cost a fortune for Muji cotton. Think I got a 190 pad pack for less than $8.00 USD and I've given some away, still got plenty.