Here's the sizes for dogs:
Dog's Apparel
Dog T-Shirt
Section Small Medium Large XL 2XL
Full Body Length 10" 12" 15.5" 17.5" 20"
Neck Opening 4" 4.75" 7.75" 8.75" 9.5"
Sleeve Length 4" 4" 5.5" 6.5" 7.5"
Sleeve Opening 2" 2" 2.75" 3" 3.125"
Chest Width 7.25" 8.25" 9.25" 11.5" 12.75"
http://www.cafepress.com/cp/info/hel...chart.aspx#110