You'd have to figure it by measurements. Here's a chart
for baby measurements, you'd need to add a little more for some ease. Lion has a baby hoodie
that's knit cuff to cuff which would give you some numbers to work with. I made it and left off the hood because I didn't have enough; the hood is knit separate and sewn on anyway. The sleeves are a bit short I think, I knit it big enough for a size 2 and made the sleeves long enough to roll up a little so my GD can wear it a bit longer.