2004 X5 4.4i tire size and rim question

[crazy_camaro]

Hi. I have a 2004 X5 4.4i that and I need to change tires. the car comes with 255/50/19 in front and 285/45/19 in the back. I was thinking about Atturo AZ800. My question is what is the widest I could go in the back with my stock rims? My guy at the shop told me I could put 305s in the back. is that possible? not sure what the stock rim size is (probably 10 inch). Can someone give me their input? I like the look for the wide tires in the back.
Also if anyone has the Atturo tires installed, please tell me about your experience with these tires.

Thanks in advance