A lot of it depends on what wood you're using, if I'm using a softer wood, I like the board to be a little bigger so I know that I'll have enough wood to make a coal before I just burn a hole through the board. In general, I like mine around a half inch in thickness. For the spindle, it's personal preference. I've found that I get more friction with a wider one, but they seem to take more energy to move than a thinner one. I usually like mine about as big around as a magic marker, nothing bigger around than your thumb. I also make my spindles in the shape of an octagon and not round. That way the string from the bow drill grips it better, but if you use rawhide or a real "grabby" kind of cordage for the string then there isn't much difference between an octagon shaped spindle and a round one. If you have a really smooth textured string, you can braid it to get it to grab the spindle better too.
For the tender, I really like cattail fluff. I put that inside some some cedar or juniper bark shavings. It looks like cattail fluff inside a bird's nest but it catches fire from a coal very quickly. I also let my coal "breathe" for a few seconds (just fan it with your hand while it's still near the hearth board) so it can become more stable before I put it in the tender bundle.
