@rs89 – I’m 6’4″ and quite broad/athletic and I wear an L. It’s nice and stretchy. There’s room to wear it over a heavier jersey in Winter but I wouldn’t want to size down.
For reference, it fits like a Castelli Gabba in L.
I wear a L in Castelli and Rapha too if that helps you place the sizing.
It’s a good piece of kit, especially for riding on the road. Only downsides I’ve found:
– Sometimes it can be too warm (but this might be down to my layering
– The armholes are a bit small (but stretchy)
– Don’t wear it for activities off the bike (running, etc). It has a high collar with the reflective material on which is fine when your head’s fairly static, but if you’re running and moving your head constantly, it chafes the jaw.