I think the spare on my 06 MC is a 15" but I could be wrong. Partsman is right though so long as its close it will be fine.....just dont leave it on there for an extended period as that is why its called a "spare"....not for extended use.
The standard 15" wheels will clear the S brakes on the non-R56 cars. Mine for example:
Like Partsman mentioned, the overall diameter of the tire is what needs to be close to stock, not the wheel diameter. A 15" wheel with 175/65/15 will have nearly the same overall diameter as a 16" wheel with 205/50/16.
Will a 15" wheel fit our brakes? The answer is 'maybe'. It depends on the shape and offset of the wheel. As someone mentioned above, the 15" will fit more easily on the back wheels (smaller brakes), so if it won't fit the front when you have a front blow out, put the 15" on the back and move a good back wheel to the front.