Okay mate, I wouldn`t discourage you in your venture, but do take on board what has been suggested in the forum. I am professionally working with an entrepreneur who is exactly in the same boat as you are, check his site: www.mrcapac.com
Okay, you raised concern about fitting! I dn`t think its that much a problem.Sizes are standards these days and as long as your quality is fine you shouldn`t have problems.
Now, this MrCapac T-shirt site has a tight theme and market. It targets its audience, which is sea-surfing and outdoor sports community. So you too have to define your niche and know the market you are serving.
It can happen just go for it! We are here to help!