Gauge
16sts = 7.25cm in stranded st st on 3.75mm needles
should be 6cm
use 3.25mm instead

Size
Hat pattern says 55 cm / 21.5”
Adult woman medium body measurement is 22.5”
Adult woman hat medium hat size is 20.5-21.5”; probably less stretchy with stranded colorwork so 1” is best so 21.5”
Perfect!
55cm x 27sts/10cm = 148.5sts
CO 144 because motif = 16 and 16x4 = 144

I did one extra diamond repeat for 3 diamonds total.

Yarn remainders
14g mohair
14g black
31g white