This guy put 18" wheels but it was close. Doesn't look like he ran staggered though.

http://www.xoutpost.com/articles/x5/...ter-setup.html

Do a search and I'm sure you'll come up with different combinations.

You can just test fit the Z4 wheels and see how it goes. If they fit, then just run the same size tires on your staggered wheels. Just because you have staggered wheels doesn't automatically mean you have to run different sized tires.

Good Luck!