Why BMW sold only 18 winter tyres for 640xd?



I went to a BMW dealer to buy winter tyres for my 2015 BMW 640xd Gran coupe M-package. I wanted to buy 19 but he told me that they are offering only 18 for winter? Why is that?

Does anyone have any idea if i could put 19 winter tyres and whether it should be a different size front and rear, considering that the car is x-drive?