The pattern calls for fingering weight yarn (4 ply) held double but I really dislike holding yarn double unless absolutely necessary. I went with a DK weight (8 ply) and achieved gauge pretty closely. I do wish however that I had used a sport weight (5 ply) in the same gauge for a little more drape. My yarn is a little stiff.
The yarn is Yarnundyed Merino which I dyed myself in a variegated purple.
I knit the 20" size (and my head measures 22") and the dimensions came out exact but to me it does seem a little big. On the flip side, it will easily cover my ears on those bitterly cold days that are headed my way.