This pattern can be found in my 2016 book Double or Nothing: Reversible Knitting for the Adventurous.
In my Craftsy class, I released a hat called Atyria. I was never particularly happy with it, but it served its purpose. When I realized that the rights had reverted back to me, I took the opportunity to do a radical redesign, both adding complexity and simplifying the underlying techniques. The end result is Atyria II.
The pattern is supposed to evoke an organism, something between animal and plant, that shows the growth cycle of those spirals I’ve been playing with in other patterns.
A 19 (21, 23)” hat will be 8.25 (8.25, 8.5)” tall. For the 21” size, the finished piece uses a total of 80 g of yarn, which translates to about 35% of the total yardage you’ll have. There should be no problem making the larger size, even if you wanted to work some extra height into the pattern.