And the measurements chart here is not much help.
I typically wear a size Medium in dress shirts and t shirts. All of my bike jerseys are Larges (some a little looser than others but nothing flapping around).
A size Large in this Mavic shirt is so massive could double as a sun tent for an entire family picnic, or a good parachute for a sky-diver. The Medium is better suited to my massive Shrek-sized biking buddy, or two of me... I'm going to give the small a shot next, but paying for return shipping (again) is making this sale price seem like a bit less of a deal.