Determining sizing for children's clothing is difficult, given that brands don't always fit the same for every child.
I generally use Geranimals brand shirts or Rabbit Skins brand. The Geranimals seem to run true to size however Rabbit Skins run about a size to two sizes small. When ordering, indicate the size your child normally wears and I will let you know which brand I'll be using so we can plan the size accordingly.
The sizing info for Rabbit Skins brand is as follows:
Rabbit Skins brand
Sizing Information
(in inches) 6M 12M 18M
BODY WIDTH 9 3⁄4 10 1⁄4 10 3⁄4
FULL BODY LENGTH 12 1⁄2 13 13 1⁄2
SLEEVE LENGTH 3 1⁄4 3 1⁄2 3 3⁄4
Sizing Information
(in inches) 2T 3T 4T 5/6 7
BODY WIDTH 12 13 14 15 16
FULL BODY LENGTH 14 15 16 18 1⁄2 19 1⁄2
SLEEVE LENGTH 4 4 1⁄4 4 1⁄2 5 5 1⁄4
For Geranimals brand, you can find sizing information here:
http://www.garanimals.com/size_charts.htm