Best thing to do with any brand of jersey you plan on wearing, is to get a measurement. I've got a number of M&N hoops jerseys and they're all over the map.
Ask for a measurement across the chest and see where you're at. Compare that to a measurement on a jersey/shirt you already have with the desired fit.

24" = true 48
25" = true 50
26" = true 52

You get the idea. Forget about the letter sizing.