I'm learning from experience. Using an 8 dent heddle, I used worsted weight for the warp and Isaac Mizrahi Craft Sutton for the weft.
Sutton is a super bulky yarn (#6). I beat the weft fairly firmly and ended up with quite a dense piece of fabric,
which is why I call this piece My Horse Blanket. It's very pretty, but a bit heavier than I would normally choose for a scarf.