Size Does Matter

I think it might vary with your market. Some markets might not need any smalls, some might not need any XXLs.

I don't think there is a industry wide standard

I've always had Medium/Large as the core market, although many say Large/x-large.

the top selling fashion item type shirts/garments are in junior/girly sizes.....
Adult Unisex Box Type T-Shirts M-L-XL sell good.

but it depends on your market I guess.

We are a country of large people and getting larger. Also most fashion trends, with the exception of womens clothing, suggest that people like wearing oversized clothing, but ...... take into consideration the art and how it would show on a person who is wearing it on a shirt that is too big for them.