You will need both size US 3 (3.25 mm) and US 6 (4.0 mm) needles.

Finished sizes:
0-6 months: 14” (36 cm) around x 6” (15.25 cm) tall
6-12 months: 18” (45.5 cm) around x 7” (17.75 cm) tall

Yarn used:
Rowan Baby Merino Silk DK
A. Dawn #672 - 1 ball
B. Iceberg # 699 - 1 ball