I ordered S-sized (EU) jersey, which is my normal clothing size and it turned out to be the right decision.
For me, the jersey is quite tight-fitting but does not cause any movement restriction or discomfort. In general, Rapha Men’s Core Jersey fits very well across the chest, waist, and shoulders.
The only slight issue I found is the baggy sleeves. I wish the sleeves would be snugger.
Scoring it a 9/10 for the fit.