From the Knit Picks Website:
This sweater is designed for those incredible growth spurts, which occur in boys in their tween and teen years. The rib knit is very elastic, the sleeves are nice and long and can be rolled up at first. By using the washable Merino, boys can wear this sweater just about anywhere and it can be machine washed.

Needles:
Size 5 (3.75mm): 24” circular & 16” circular & DPNs Size 3 (3.25mm) or Size 4 (3.5mm) 16” circular & DPNs