Two Versions in book:

Blue Version
Gauge: 20 stitches and 45 rows = 4 inches in garter stitch
Needles: US4
Yarn: Alchemy Yarns Bamboo

Green Version
Gauge: 19 stitches and 40 rows = 4 inches in garter stitch
Needles: US5
Yarn: Blue Heron Yarns Cotton/Rayon Seed

From book:
“This collar is what those single skeins were meant for… . it curves and lies flat all at the same time… .You can knit it looser than ball band gauge and let the front flow into ruffles… Or knit it at ball band gauge or snugger and go for the sleek and tailored look.”