I'm 6'1" @ 158lbs and I wear a Medium or Large in the Castelli jerseys I have. With Hincapie or PI, I wear medium and those flap in the wind a little around the shoulders and gut. The Castelli is certainly the tightest, but that was the point of me ordering that specific Castelli so I'm ok with that. I can fit into small PI/Hincapie, but they aren't long enough. It blows my mind - just how big are americans that a 6'1" dude can fit in a small?!