Here is the size chart from the package (chest size in inches):
small = 35-37
medium = 38-40
large = 41-43
XL = 44-46
XXL = 47-49
XXXL = 50-53

The product description says the fabric is polyester French terry. I'm not sure what that means, since the fabric has a smooth woven surface on both the outside and inside.

As shown in the photo, this is not a baggy fit, like casual T-shirts. It is not a compression form-fit, either. The shirt is definitely not skin-tight, but it is somewhat contoured to improve sweat wicking and reduce flapping in the wind and the risk of snagging on gym equipment. The fabric has plenty of stretch to avoid binding and dries quickly so that it doesn't get saturated with sweat when you're working out. Fabric and construction are excellent for the current $12-$15 price. It does not snag as easy as cheaper workout shirts and the hems are more sturdily sewn so they don't come apart after a few washings.