My head is 23” and the small / medium fit perfectly (US7 needles).
I reknit this because I thought I’d need a large and I could tell half-way through it would be far too big.
A couple of stitch markers helped make following this cart much easier, and once you get the hang of it, it’s a fast, pretty brainless knit. Very clever construction. I may or may not have stayed up until 2am to finish because I felt like I was “sooooo close!” for the last two hours. Feeling the consequences of sleep deprivation big-time today, but I love the hat.