What size authentic jersey?

i am gonna buy an authentic Kevin Garnett jersey... i have 11 celtic swingman jerseys that are XL's they fit nice.. what size in an authentic would be closet to an XL swingman? cuz i know on the bottom of swingman jerseys it says like "L" "XL" or "2XL" and so on.. and authentics just say like "48" 52" and so on/