I got one of these last time. The description says “Slim-fit design - for a generous fit, go one size up,” but I was disappointed that it was not what I would consider slim fit; it seemed like a normal shirt, and was even a little on the baggy side. I ordered a medium (which is what I normally wear) according to chest measurement, and it fit well in the chest, but it was too big and billowy around the waist. It seemed like the shirt was cut for someone with a bit of a beer belly. If I had ordered a size up, it would have been way too big. I should note that I’m not some bodybuilder type with a huge chest and tiny waist, I’m pretty average shaped.
In conclusion: ok shirt, but not slim fit.