Measurements
Each helmet is designed for children and teens of chest size:
24, 26, 28, 30, 32 inches. (therefore of approximate age 5 to 15)

Each pattern has a different set of recommended yarns. The following is from the Maryland version.

Materials
ARAN KNITTING
Maryland Aran Knitting
2, 2, 2, 2, 3 2 oz. balls
Maryland Lightweight Bulky Knit
2, 2, 2, 2, 3 2 oz. balls
Stitch holders
2 buttons
Cable needle.