Right, I have 3 Wilson jerseys, 42, 44 and 46. In the current Reebok authentic range I wear a 48 or 50. The 42 is too small for me and my paunch, the 44 and 46 are good. I would guess that if you are a medium replica or 48 authentic the 44 or 46 should be ok.
I have a Russell 48 which is the same as the Reebok size 48.
NJFP Merchandise available at http://www.zazzle.co.uk/ukbillsfan