Tshirt size Query

[i_am_tre]

Okay, so, say I wanted to get Northern Downpour membership. I was looking at the Sign up bit and where it says tshirt size it just says Youth Medium, Youth Large, Small, Medium, Large, Extra Large, Extra Extra Large.
Question is: What are the sizes for these tshirts? Inches will do if anyone can help.

Thanks.

Edit: Im normally a Large which is 40-42" (i think thats across) so yeah...

-Jimmy