I really like cotton, and silica tastes a little off for me now. But I think its a personal taste. The biggest pro for me to originally try cotton was that I could just run down to walmart and get the peaches n cream 100% cotton yarn 360ft for like $3 and didn't have to wait on vape mail again. I know my buddy uses the cotton ball stuff, I hate touching it, but it works well for him. So I would at least try the cotton and you could order some silica or ekowool or whatever. (this is where its good to have local vaping friends to share with)
As for size if you use 1/16 drill bit like I do that works out to 1.58mm. So that really depends on the size coils you are making.
Also just make sure if you do use cotton to make sure the wick is wet before you hit that fire button. Cotton will burn, a taste you won't soon forget. Oh and when you boil some of the yarn cut it into smaller pieces first so you don't end up with a giant knot.
Best of luck.