The best way is to take one of your existing
t-shirts, lay it flat, and then measure the
width and length. Or, know the persons chest
and height measurements. We use 100% cotton GILDAN tshirts.

 
Gildan Activewear
 Adult
Width
Length
   
Small
18"
28"
Medium
20"
29"
Large
22"
30"
X-Large
24"
31"
XXL
26"
32"
XXXL
28"
33"
4XL
30"
34"
5XL
32"
35"
   
Youth
Width
Length
   
X-Small (2-4)
16"
20"
Small (6-8) 
17"
22"
Medium (10-12)
18"
23.5"
 Large (14-16)
19"
25"