Size 18 Superlon is the string I use most often for beaded kumihimo. Perfect fit with size 8/0 Japanese seed beads. I use it doubled with size 6/0 seed beads.
Pick a color to match your seed beads and it will blend into your braid.
When braiding with Superlon by itself with no beads, an 8-warp round braid (kongoh gumi) with one piece of superlon per warp will yield a braid about 2.7mm thick.
Superlon goes by many names:
Twisted Nylon Braiding String
There are approximately 77 yards per spool.