Are you an adult?

If you don't know your shirt size,
why the hell are you on this site?
PHACOUGH!

Take a look in your drawer and see what size your wife buys you!
These are t-shirts not dress shirts, they can and will have some stretch and give to them along with some shrinkage as you wash them.. mostly Cotton blends.

If you cant figure out how to pick your shirt size,
ask your mommy.