design a t-shirtWhat is are good size measurements for a t

I am designing a graduation t-shirt for my 8th grade class and I have no idea what size i should make it.
THe designs I sketched fill up a whole piece of paper each for the front and back design.
T-shirt sizes will be adult small- adult XL

Please help, any suggestion appreciated
Smalldesign a t-shirt Chest:18'' Length:28''
Medium Chest:20'' Length:28''
Large Chest:22'' Length:30''
and Extra Large Chest:24'' Length:30''

Sizes are in inches. You should probably take a survey or ask everyone that is getting a t-shirt to request their size.

A really easy way to get your design on a good looking t-shidesign a t-shirtrt is to order it online. That way you won't have to worry about messing up the measurements. A good site to order from is cafepress:
Possibly 6 x 6 would meet your need for all the sizes.