From the Knit Picks Website:
It’s party time! Boxy jacket finishes this adorable outfit; sizes from3 months to 8 years! Color chart included to create polka dots.

Sizes:
3-6 months - 20” chest
1 year - 22” chest
2 years - 24” chest
4 years - 26” chest
5 years - 28” chest
6 years - 29.5” chest
8 years - 31” chest

Needles:
Size 4 (3.5mm): 16” circular
Size 4 (3.5mm): long circular needle