Size numbers 6-8, 10-12, and so on are NOT standard. Each manufacturer is allowed to cut the garment to any size they desire. While there is good consistency in adult sizes, youth shirts vary widely in size.
A better measure is the actual width and length of the garment. We have a chart online for our most popular garments here: http://www.classb.com/help/305/t-shirt-size-charts
We carry over 100 different styles and brands of garments. On all styles of garment there is a chart with the manufacturers number size. Please contact us if you have a specific size question.
Visit this link for our most popular t-shirt styles: Custom T-shirts Screen Printed from ClassB.com