Gauge is 15 stitches =2 inches
10 rows = 1 inch

Mistral has notes :

Top-Shaping should read:

Row 1: K 48 (51, 54) sts; sl 1; K 1, Psso; turn.

Row 2: S1, P 24 (26, 28) sts; P 2 tog, turn.

Row 3: S1, K 24 (26, 28) sts; S1 , K1, Psso; turn.

The M with this yarn & 3.25mm needles fits my son, who is 9 months old with a bit of growing room. He normally wears shop bought hats in size 2years ish.