Goalkeeper jersey

Neither really. It's basically in the middle but a little closer to authentic fit. It's the weight and material of the authentic but the fit is mostly loose. It's tapered under the arms and around the wrist.
 
The keeper jerseys are actually the player issued version. Player issue Adidas jerseys are sized by number and not regular size. I have a size 8 and I'd say it's between an authentic L and XL.