My son is wearing a size 4 and he is about average for chest size so he has about 2.5 inches of positive ease.

Sizes
12m [18m, 2, 4, 6, 8, 10]

Finished measurements
Chest: 20.5 [21.5, 23.5, 25.5, 27.5, 29, 31.5] inches
Length: 13 [14, 15.75, 16.25 18.25, 19.75, 21] inches

Materials
The Verdant Gryphon Traveller [100% Superwash
Merino; 280 yards per 113 grams (3.99 oz) ]
colorway: Smuggler’s Notch 2 [2, 3, 3, 3, 4, 4] skeins.

Needles
US5/3.75mm 16 or 24 inch circular
US5/3.75mm double pointed needles
or needle size to obtain gauge

Notions
Stitch markers-4
Stitch holders/scrap yarn
1 button (half inch)
1 toggle button

Gauge (washed and blocked)
22 sts and 28 rows in 4 inches of stockinette stitch
21 sts and 34 rows in 4 inches of garter stitch