Great aero fit and super comfy!

5'10", 160 lbs with long and lanky arms. The large is comfortable. I especially like the Euro Pro sleeves.

Haute qualité

I'm 6'1", 185 lbs with a 42" chest on a good day. I got the XL and the fit is about right. If I had sized down it'd be really skin tight; as it is I can see a bit of bunching in the fabric sort of like as it appears in the product photos. Generally comfortable stuff down into the low 60s with a base layer and the arm and waist grippers are effective at keeping everything in place while you're in riding position.

If I'm nitpicking I have two gripes:
(1) the hole for your head is a bit oddly positioned. Maybe it's just my build but I feel like if I let it sit the way it wants to sit on my shoulders, the zipper digs into my neck a little bit. Not that it's uncomfortable; it just looks off. And if I adjust the jersey so the zipper is where I feel like it needs to be, I get some bunching in the chest and stretching across the shoulders. This is of course exacerbated when in the riding position.
(2) The color is more saturated and vibrant than it looks in the product photos. Whereas I was hoping for a kind of earth-tone army green type color, it looks straight up green in person.

Sleeve deaign is terrible. They seemed okay with try on before washing upon receiving. They ride up your arms while cycling and meshy material is stretchy and has changed form after 1 cold wash. They will not accept returns after use which I understand I guess but I will never order another piece of apparel from them.