Yarn weight
Bulky / 12 ply (7 wpi) ?
This is a “recipe” which is adaptable to any head size, in any yarn (though worsted to bulky weight wool is probably ideal). I knit the prototype in KnitPicks Suri Dream held double, and a second hat in Elann’s Peruvian Cuzco (alpaca) with stripes of Rowan Big Wool Tuft.